VTFT with a top-gate structure

ABSTRACT

An electronic device includes a vertical-support-element on a substrate. The vertical-support-element extends away from the substrate and includes a first edge having a first reentrant profile. A conformal semiconductor layer is in contact with the vertical-support-element in the reentrant profile. A first electrode is in contact with a first portion of the semiconductor layer and is located over a top of the vertical-support-element. A second electrode is in contact with a second portion of the semiconductor layer and is located over the substrate and not over the vertical-support-element. The second electrode is adjacent to the first edge of the vertical-support-element. A conformal dielectric layer is on the conformal semiconductor layer in the reentrant profile. A conformal conductive gate is on the conformal dielectric layer in the first reentrant profile. The first electrode and the second electrode define a semiconductor channel of a transistor.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned, U.S. patent application Ser. No.14/737,544, entitled “VERTICAL AND PLANAR TFTS ON COMMON SUBSTRATE”,Ser. No. 14/737,560, entitled “BOTTOM-GATE AND TOP-GATE VTFTS ON COMMONSTRUCTURE”, Ser. No. 14/737,577, entitled “DUAL-GATE VTFT, all filedJun. 12, 2015.

FIELD OF THE INVENTION

This invention relates generally to semiconductor devices and inparticular to transistor devices.

BACKGROUND OF THE INVENTION

Modern-day electronics systems require multiple patterned layers ofelectrically or optically active materials, sometimes over a relativelylarge substrate. Electronics such as radio frequency identification(RFID) tags, photovoltaics, and optical and chemical sensors all requiresome level of patterning in their electronic circuitry. Flat paneldisplays, such as liquid crystal displays or electroluminescent displaysrely upon accurately patterned sequential layers to form thin filmcomponents of the backplane. These electronic components includecapacitors, transistors, and power buses. The usual combination ofphotolithographic patterning methods and selective etch processes hasseveral shortcomings including high cost, difficulty with largesubstrates, and complexity of selective etch processes.

The feature size obtainable using traditional processing methods islimited by the resolution of the photolithography tools. Currently theminimum feature size for large area display backplanes is around amicron, and requires expensive high end equipment. Minimum feature sizesfor large area substrates with less expensive equipment can be muchlarger. High speed circuit operation requires thin film transistors(TFTs) with high drive current, and many applications additionallyrequire that the drive current be obtained with low voltage operation.It is well known that TFT performance is improved by reducing thechannel length. To move beyond the exposure limitation of feature size,vertical transistors of various architectures are currently beingstudied. In a vertical TFT (VTFT) architecture, the channel is formedperpendicular to the substrate, and therefore the channel length (L) canbe controlled by the height of a layer in the transistor.

Recent work in the fabrication of VTFTs, while yielding short channellength devices, has used otherwise standard photolithographic techniqueswith complex semiconductor processes. For example, since it is notcurrently possible to put patterns directly on walls which are verticalwith respect to the substrate surface, vertical wall patterning has beenaccomplished using a suitable temporary filler material to partiallyfill in a trench. The temporary filler material acts as a mask for theportions of the wall located underneath while allowing for processing ofthe walls above the temporary filler material. This has been used, forexample, when an oxide is to be deposited exclusively on vertical wallsbelow a temporary filler material, where the oxide is first deposited orproduced over the entire surface of the relief. The relief or trench isinitially completely filled with a suitable temporary filler material.Then, the temporary filler material is recessed back to a depth thatjust covers the desired oxide. After uncovered sections of the oxide areremoved, the remaining temporary filler material is removed.

Alternatively, when it is necessary that an oxide be deposited orproduced only in upper regions of a vertical wall, an etching stoplayer, for example a nitride layer, is first provided over the entiresurface of the entire relief pattern. A different material, susceptibleto directional etching, for example polycrystalline silicon, is used tofill the relief, and is etched back as far as the desired coverage depthof the final vertical oxide. After the etching stop layer is removedfrom the unfilled sections of the walls, an oxide is deposited orgenerated using a thermal technique in the uncovered regions. Next, theoxide is anisotropically etched which removes the deposited oxide fromhorizontal. This is followed by removal of the filler material and thenthe removal of the etching stop layer.

In light of the complicated existing processes there is an ongoing needto provide semiconductor device architectures that include patternedvertical or inclined device surfaces. There is also an ongoing need toprovide simple manufacturing techniques capable of processing smalldevice features of semiconductor devices without requiring highresolution alignments and small gap printing for vertical TFTs. There isalso an ongoing need to provide higher current semiconductor devices byimproving the series resistance of the device.

To maintain good device performance when shrinking the size of thechannel, it is typical to scale the layer thicknesses with the size ofthe device. For example, conventional production CMOS processes withchannel lengths of 90 nm and lower often utilize dielectric layerthicknesses of less than 10 nm. While there are many processes todeposit dielectric materials, few result in high quality films at thesethicknesses. Atomic layer deposition (ALD) is a process that is bothconformal and known to result in high quality thin layers when used withoptimized process conditions.

There is growing interest in combining ALD with a technology known asselective area deposition (SAD). As the name implies, selective areadeposition involves treating portion(s) of a substrate such that amaterial is deposited only in those areas that are desired, or selected.Sinha et al. (J. Vac. Sci. Technol. B 24 6 2523-2532 (2006)), haveremarked that selective area ALD requires that designated areas of asurface be masked or “protected” to prevent ALD reactions in thoseselected areas, thus ensuring that the ALD film nucleates and grows onlyon the desired unmasked regions. It is also possible to have SADprocesses where the selected areas of the surface area are “activated”or surface modified in such a way that the film is deposited only on theactivated areas. There are many potential advantages to selective areadeposition techniques, such as eliminating an etch process for filmpatterning, reduction in the number of cleaning steps required, andpatterning of materials which are difficult to etch.

SAD work to date has focused on the problem of patterning a singlematerial during deposition. There persists a problem of combiningmultiple SAD steps to form working devices. Processes for buildingcomplete devices need to be able to control the properties of thecritical interfaces, particularly in field effect devices like TFTs.There remains a need for novel processes to simplify the manufacture ofvertical TFTs, as well as a need for novel processes that use SAD anddigital patterning processes to pattern devices which have criticalvertical features, such as VTFTs.

A particularly useful electronic device in building functional circuitryis an inverter, which functions to invert the polarity of an inputsignal. In CMOS circuitry, inverters are typically easy to design butdisadvantageously expensive to produce and utilize complicatedproduction processes. It is possible to build all NMOS or PMOSinverters. However, particularly for enhancement-depletion mode circuitsthere are challenges to independently controlling the behavior of eachtransistor in the inverter circuit. Typically, the depletion modetransistor will have a thicker semiconductor layer than the enhancementmode transistor, increasing process complexity and increasing cost.Other alternatives include using dual gate architectures or multilayersemiconductor stacks, which have similar issues of process complexityand cost. As the industry endeavors to build circuitry using printingmethods, individual transistor size has a direct impact on the overallcircuit footprint, as the individual component transistors are sizedusing their channel dimensions. There remains a need to build highquality inverters using simple processes, by employing novelarchitectures to control individual transistor, and therefore, circuitperformance.

More generally, there is a need in the industry to have a flexibleprocess that enables facile fabrication of the components of anyelectronic circuit. Processes required to form short channel deviceshave previously been incompatible with processes used to form longerchannel devices. There is a need to be able to mix and match componentsfor the best circuit design—rather than being limited by thecapabilities of conventional processes.

SUMMARY OF THE INVENTION

According to an aspect of the invention, an electronic device includes asubstrate, and a vertical-support-element on the substrate, thevertical-support-element extending away from the substrate, thevertical-support-element including a first edge having a first reentrantprofile. A conformal semiconductor layer is in contact with thevertical-support-element in the reentrant profile. There is a firstelectrode in contact with a first portion of the semiconductor layer,the first electrode located over a top of the vertical-support-element,and a second electrode in contact with a second portion of thesemiconductor layer, the second electrode located over the substrate andnot over the vertical-support-element, and adjacent to the first edge ofthe vertical-support-element. A conformal dielectric layer is on theconformal semiconductor layer in the reentrant profile, and a conformalconductive gate is on the conformal dielectric layer in the firstreentrant profile. The first electrode and the second electrode define asemiconductor channel of a transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the example embodiments of the inventionpresented below, reference is made to the accompanying drawings, inwhich:

FIGS. 1a and 1b are a cross-sectional view and plan view, respectively,of one embodiment of the top-gate vertical transistor of the presentinvention;

FIGS. 2a through 2d are cross-sectional views of alternative embodimentsof the top-gate vertical transistor of the present invention;

FIGS. 3a through 3c are schematic cross-sectional views of alternativevertical-support-elements of the present invention having reentrantprofiles;

FIGS. 4a through 4f are various examples of vertical-support-elementshaving reentrant profiles formed using a post and cap;

FIGS. 5a and 5b are a cross-sectional view and plan view, respectively,of another embodiment of the top-gate vertical transistor of the presentinvention with lower overlap capacitance;

FIGS. 6a and 6b are a cross-sectional view and plan view, respectively,of two independently operable top-gate vertical transistors on the samevertical-support-element of the present invention;

FIGS. 7a and 7b are a cross-sectional view and plan view, respectively,of another embodiment of a single top-gate vertical transistor on avertical-support-element of the present invention;

FIGS. 8a and 8b are a cross-sectional view and plan view, respectively,of a bottom-gate transistor on a vertical-support-element;

FIGS. 9a and 9b are a cross-sectional view and plan view, respectively,of a top-gate vertical transistor and a bottom-gate transistor on thesame vertical-support-element of the present invention;

FIG. 10a is a schematic diagram of a circuit;

FIGS. 10b and 10c are a cross-sectional view and plan view,respectively, of a top-gate vertical transistor and a bottom-gatetransistor on the same vertical-support-element operable as anenhancement-depletion mode inverter having the equivalent circuit ofFIG. 10 a;

FIG. 11 is a cross-sectional view of a top-gate vertical transistorhaving a thicker gate dielectric than a bottom-gate transistor on thesame vertical-support-element operable as an enhancement-depletion modeinverter;

FIGS. 12a and 12b are a cross-sectional view and plan view,respectively, of two dual-gate vertical transistors on the samevertical-support-element of the present invention;

FIG. 13 is a cross-sectional view of two independently operabledual-gate vertical transistors on the same vertical-support-element;

FIGS. 14a and 14c are a cross-sectional view and plan view,respectively, of two dual-gate vertical transistors on the samevertical-support-element operable as an inverter having the equivalentcircuit of FIG. 14 b;

FIG. 14b is a schematic diagram of the equivalent circuit of thedual-gate transistors shown in FIGS. 14a and 14 c;

FIG. 15a is a schematic diagram of a circuit;

FIG. 15b is a cross-sectional view of two bottom-gate verticaltransistors on the same vertical-support-element operable as anall-enhancement-mode inverter having the equivalent circuit of FIG. 15a;

FIG. 16 is a cross-sectional view of an alternative embodiment of anall-enhancement-mode where the load TFT has a thicker dielectric thanthe drive TFT;

FIGS. 17a and 17b are cross-sectional views corresponding to the planview shown in FIG. 17c of a typical prior art top-gate planar TFT;

FIG. 17c is a plan view of a typical prior art top-gate planar TFT shownin FIGS. 17a and 17 b;

FIG. 18a is a cross-sectional view corresponding to the plan view shownin FIG. 18b of a typical prior art bottom-gate planar TFT with coplanarcontacts;

FIG. 19a is a cross-sectional view of the plan view shown in FIG. 19b ofa typical prior art bottom-gate planar TFT with staggered contacts;

FIGS. 20a through 20d are cross-sectional views of alternativebottom-gate planar TFTs having different dielectric layerconfigurations;

FIG. 21 is a cross-sectional view of an electronic element of thepresent invention with a bottom-gate VTFT and a bottom-gate planar TFTon a common substrate having the same material layers;

FIG. 22 is a cross-sectional view of an electronic element of thepresent invention with a bottom-gate VTFT and a top-gate planar TFT onthe same substrate;

FIG. 23 is a cross-sectional view of an electronic element of thepresent invention with a top-gate VTFT and a bottom-gate planar TFT onthe same substrate having the same material layers;

FIG. 24 is a cross-sectional view of an electronic element of thepresent invention with a top-gate VTFT and a top-gate planar TFT on thesame substrate, having the same material layers;

FIG. 25a is a schematic diagram of a circuit;

FIGS. 25b and 25c are a plan view and cross-sectional view,respectively, of a bottom-gate vertical transistor and a planarbottom-gate TFT configured to be operable as an all-enhancement-modeinverter having the equivalent circuit of FIG. 25 a;

FIG. 26 is cross-sectional view, respectively, of a bottom-gate verticaltransistor and a planar bottom-gate TFT configured to be operable as anall-enhancement-mode inverter having the equivalent circuit of FIG. 15a;

FIG. 27 is cross-sectional view, respectively, of a top-gate verticaltransistor and a bottom-gate transistor on the samevertical-support-element operable as an enhancement-depletion modeinverter having the equivalent circuit of FIG. 10 a;

FIG. 28 is a cross-sectional view of a top-gate vertical transistorhaving a thicker gate dielectric than a bottom-gate transistor on thesame vertical-support-element operable as an enhancement-depletion modeinverter;

FIG. 29 is a cross-sectional side view of a deposition device, showingthe arrangement of gaseous materials provided to a substrate that issubject to the thin film deposition process of the Examples;

FIG. 30 is a graph illustrating the relationship between gate voltageand drain and gate current for top-gate VTFTs of Inventive Examples I1,I2 and I3;

FIG. 31 is a graph illustrating the relationship between input voltageand output voltage for the enhancement-top-gate VTFTs of InventiveExamples I4 and I5 and bottom-gate VTFTs BG1 and BG2;

FIG. 32 is a graph illustrating the relationship between input voltageand output voltage for the enhancement-depletion inverters of InventiveExamples I6 and I7; and

FIG. 33 is a graph illustrating the relationship between time per stageand input voltage for the ring oscillators of Inventive Examples I8 andI9 and Comparative Example C1.

DETAILED DESCRIPTION OF THE INVENTION

The present description will be directed in particular to elementsforming part of, or cooperating more directly with, an apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown, labeled, or described can take variousforms well known to those skilled in the art. In the followingdescription and drawings, identical reference numerals have been used,where possible, to designate identical elements. It is to be understoodthat elements and components can be referred to in singular or pluralform, as appropriate, without limiting the scope of the invention.

The example embodiments of the present invention are illustratedschematically and are not to scale for the sake of clarity. One ofordinary skill in the art will be able to readily determine the specificsize and interconnections of the elements of the example embodiments ofthe present invention. Therefore, the figures provided are not drawn toscale but are intended to show overall function and the structuralarrangement of some embodiments of the present invention.

Even though specific embodiments of the invention have been describedherein, it should be noted that the present invention is not limited tothese embodiments. In particular, any features described with respect toone embodiment may also be used in other embodiments, where compatible.The features of the different embodiments can be exchanged, wherecompatible.

For the description that follows, the term “gas” or “gaseous material”is used in a broad sense to encompass any of a range of vaporized orgaseous elements, compounds, or materials. Other terms used herein, suchas: “reactant”, “precursor”, “vacuum”, and “inert gas”, for example, allhave their conventional meanings as would be well understood by thoseskilled in the materials deposition art. The term “over” refers to therelative position of an element to another and is insensitive toorientation, such that if one element is over another it is stillfunctionally over if the entire stack is flipped upside down. As such,the terms “over”, “under”, and “on” are functionally equivalent and donot require the elements to be in contact, and additionally do notprohibit the existence of intervening layers within a structure. Theterm “adjacent” is used herein in a broad sense to mean an element nextto or adjoining another element. The figures provided are not drawn toscale but are intended to show overall function and the structuralarrangement of some embodiments of the present invention.

The embodiments of the present invention relates to thin filmtransistors (TFTs) with different architectures and configurations,including variable dielectric layer thickness, that can all be formed ona common substrate. The ability to easily fabricate multiple types oftransistors on a common substrate allows for freedom in circuit design.The ability to spatially control the dielectric layer thickness providesa means for adding extra protection against shorting, reducedcapacitance, and the use of dielectric layer thickness to size thevarious TFTs in a circuit, in addition to the commonly used channeldimensions, width (W) and length (L). Two types of TFTs are useful incircuit design—enhancement-mode and depletion-mode devices. Anenhancement-mode transistor is a transistor in which there is negligibleoff-current flow, relative to on-current flow, between a source and adrain at zero gate voltage; typically off-current values of metal oxideTFTs are less than 10⁻⁸ A, often as low as 10⁻¹¹ A. In other words, thetransistor device is normally off. In contrast, a depletion-modetransistor is normally on meaning that more than substantiallynegligible current flows between a source and a drain at zero gatevoltage.

When the TFT operates in an enhancement-mode, the charges injected fromthe source electrode into the semiconductor are mobile and a currentflows from source to drain, mainly in a thin channel region within about100 Angstroms of the semiconductor-dielectric interface. See A.Dodabalapur, L. Torsi H. E. Katz, Science 1995, 268, 270, herebyincorporated by reference. In the absence of a gate field the channelideally has few charge carriers; as a result there is ideally nosource-drain conduction when the device is in off mode.

The off-current in an enhancement-mode device is defined as the currentflowing between the source electrode and the drain electrode when chargehas not been intentionally injected into the channel by the applicationof a gate voltage. This occurs for a gate-source voltage more negative,assuming an n-channel device, than a certain voltage known as thethreshold voltage. See Sze in Semiconductor Devices—Physics andTechnology, John Wiley & Sons (1981), pages 438-443, hereby incorporatedby reference. The on-current is defined as the current flowing betweenthe source electrode and the drain electrode when charge carriers havebeen accumulated intentionally in the channel by application of anappropriate voltage to the gate electrode and the channel is conducting.For an re-channel accumulation-mode TFT, this occurs at a gate-sourcevoltage more positive than the threshold voltage. It is desirable forthis threshold voltage to be slightly positive, for enhancement moden-channel operation. Switching between on and off states is accomplishedby the application and removal of an electric field from the gateelectrode across the gate dielectric to the semiconductor-dielectricinterface, effectively charging a capacitor.

Advantageous enhancement mode TFT device characteristics include a largeratio of the on-current to the off-current, and a steep sub-thresholdslope. In the operation of such a TFT device, a voltage applied betweenthe source and drain electrodes establishes a substantial current flowonly when the control gate electrode is energized. That is, the flow ofcurrent between the source and drain electrodes is modulated orcontrolled by the bias voltage applied to the gate electrode. Therelationship between material and device parameters of thezinc-oxide-based semiconductor TFT can be expressed by the approximateequation (see Sze in Semiconductor Devices—Physics and Technology, JohnWiley & Sons (1981)):

$I_{d} = {\frac{W}{2\; L}\mu\;{C_{ox}\left( {V_{g} - V_{th}} \right)}^{2}}$

where I_(d) is the saturation source-drain current, C_(ox) is thegeometric gate capacitance associated with the insulating layer(dielectric layer), W and L are physical device dimensions, μ is thecarrier mobility in the zinc-oxide-based semiconductor, V_(g) is theapplied gate voltage, and V_(th) is the threshold voltage. Ideally, theTFT allows passage of current only when a gate voltage of appropriatepolarity is applied. However, with zero gate voltage, the off currentbetween source and drain depends on the intrinsic conductivity σ of thezinc-oxide-based semiconductor as well as the state of the semiconductorback channel.

The term “vertical transistor” as used herein refers to transistorswhere the source and drain that define a single channel are at twodifferent distances from the substrate surface (as measured orthogonalto the substrate). This arrangement results in vertical transistorswhere at least a portion of the channel is vertically oriented withrespect to the substrate, which is to say not parallel to the topsurface of the substrate. The vertical transistors of the presentinvention are vertical thin-film transistors, and are referred to by theacronym VTFT. Vertical transistors of the present invention includethose with portions of their channels in an arrangement that is parallelto the substrate surface, as long as they also have a portion which isnot parallel. The advantage of VTFTs in circuit design is their shortchannel lengths, typically much shorter than that is easily obtainableby techniques to form standard planar transistors.

As used herein, a long channel vertical transistor is a verticaltransistor whose channel length is more typical of that of a planar andtherefore long for a transistor having a vertical architecture. Longchannel vertical transistors have source and drain electrodes that areat different distances from the substrate (making it a verticaltransistor), but the majority of the semiconductor channel is parallelto the substrate. Long channel vertical transistors have advantages overboth planar and vertical transistors in designs where the integration ofshort channel (typical VTFTs) and longer channel TFTs is desired. Theability to use a portion of the vertical-support-element in the longerchannel vertical TFT construction allows for the reduction of circuitfootprint over mixed vertical and true planar TFTs.

The term “planar transistor” as used herein refers to transistors wherethe source and drain electrodes which define a single channel are at thesame distance from the substrate surface (as measured orthogonal to thesubstrate). This arrangement results in planar transistors where thechannel is parallel to the substrate surface.

Transistors of the present invention are useful in building logic gates.The term “logic gate” has the commonly understood definition. Logicgates include the AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates. A NOTgate is also called an inverter, and is referred to as such herein.Although the inverter is used as a representative logic gate to describehow the transistors geometries of the present invention can be useful incircuits, it should be understood that the transistor architectures ofthe present invention are equally useful in constructing other types oflogic gates.

The particular process sequence for fabricating inorganic thin filmtransistors and electronic devices from semiconductor, dielectric,conductive materials is determined by the structure of the desiredtransistor. There are four primary useful structures for thin-filmtransistors. In the production of a field effect transistor in aso-called inverted structure, a gate electrode can be first deposited ona substrate, for example a vacuum- or solution-deposited metal ororganic conductor, or an ALD-deposited conductor. The gate electrode isinsulated with a dielectric (often referred to as the gate-dielectric),then source and drain electrodes and a layer of the inorganicsemiconductor material are applied on top, such that the source anddrain electrodes define the length of the channel in the transistor. Thesource and drain contact geometry is referred to as coplanar.Alternatively, in a second structure, a gate electrode is depositedfirst, followed by a gate-dielectric, the semiconductor is applied, andfinally the contacts for the source electrode and drain electrode aredeposited on the semiconductor layer, again defining the length of thechannel in the transistor, and resulting in a staggered contactgeometry. These transistors where the gate is between the channel in thesemiconductor layer and the substrate, sometimes referred to as“inverted”, are referred to as “bottom-gate” transistors herein.

In an alternative third structure, the source and drain electrodes aredeposited first, then the semiconductor is deposited, and finally thedielectric and gate electrode are deposited on top—resulting in astaggered contact geometry. In an alternative fourth structure, thesemiconductor is deposited first, prior to depositing the source anddrain electrodes—resulting in a coplanar contact geometry. Thesetransistors where the gate is in on the side of the semiconductoropposite the substrate, are referred to as “top-gate” transistorsherein.

In most embodiments, a field effect transistor includes an insulatinglayer, a gate electrode, a semiconductor layer including an inorganicmaterial as described herein, a source electrode, and a drain electrode,wherein the insulating layer, the gate electrode, the semiconductorlayer, the source electrode, and the drain electrode are in any sequenceas long as the gate electrode and the semiconductor layer contactopposite sides of the insulating layer and the source electrode and thedrain electrode both contact the semiconductor layer. Both planar andvertical transistors can have bottom- or top-gate architectures, withstaggered or coplanar contacts. In the case of a vertical top-gatetransistor, the gate is on the side of the semiconductor opposite thevertical-support-element.

The vertical transistors of the present invention are formed using avertical-support-element. As used herein, the vertical-support-elementcan be any structure which has a reentrant profile. Thevertical-support-element can be uniform in material composition, or beformed of multiple materials. The vertical-support-element can also bereferred to as a post, as long as the post has the required reentrantprofile. The vertical-support-element can be formed using a post andcap, where the cap extends beyond the wall of the post to define thereentrant profile. The vertical-support-element has a height extendingaway from the substrate. Preferably, the height dimension of thevertical-support-element is less than 10 microns. U.S. application Ser.Nos. 14/198,628; 14/198,630; 14/198,631; 14/198,647; 14/198,652;14/198,658; and 14/198,664, filed Mar. 6, 2014, are all related todevices containing and methods of forming vertical-support-elementsusing structural polymer posts with an inorganic thin-film cap and arehereby incorporated by reference. The vertical-support-element can beformed from any material, including both conductive and insulatingmaterials. For vertical top-gate architectures it is preferred that thevertical-support-element be an insulating structure, that is to sayformed from insulating or dielectric materials. An electricallyconductive gate structure, as used herein, refers to the structure thatcontains the conductive material which will be used to gate the verticaltransistor channel. In some embodiments, the electrically conductivegate structure includes a vertical-support-element and an electricallyconductive gate layer.

The phrase “structural polymer” as used herein refers to the polymericmaterial used in the formation of the vertical-support-element,including the polymer post, and is additionally useful to distinguishthe structural polymer material from other polymeric materials orpolymer layers that may be used in the process. The structural polymeris a polymer that is stable in the final application, and a wide varietyof structural polymers may be used. Illustrative of structural polymersare polyesters, polyetheresters, polyamides, polyesteramides,polyurethanes, polyimides, polyetherimides, polyureas, polyamideimides,polyphenyleneoxides, phenoxy resins, epoxy resins, polyolefins,polyacrylates, polyethylene-co-vinyl alcohols (EVOH), and the like ortheir combinations and blends. The preferred structural polymers areepoxy resins and polyimides. The structural polymer can be athermoplastic polymer. The polymer can be a curable composition,including either thermal or radiation curable composition. The polymerdoes not need to be radiation curable or photosensitive, butphotosensitive formulations are useful in the present invention so longas the final cured polymer layer has the structural and mechanicalproperties required in the final application.

Polyimide is a preferred structural polymer due to the combination offilm properties such as low stress, low coefficient of thermalexpansion, low moisture uptake, high modulus and good ductility formicroelectronic applications. The rigid rod polyimide structure of CuredPI-2600 products available from Hitachi DuPont MicroSystems, forinstance, are well suited for use as a dielectric layer forsemiconductor applications. Epoxy resins are also preferred due to theirthermal and chemical properties. Radiation curable compositionscomprising a highly branched, multifunctional epoxy bisphenol A-novolacresin, such as Epon SU-8 from Momentive Specialty Chemicals Inc. is oneexample of a useful epoxy resin, although non-radiation curablecompositions are more preferred.

The process of making the vertical thin film transistors of the presentinvention can be carried out below a support temperature of about 300°C., more preferably below 250° C., or even at temperatures around roomtemperature (about 25° C. to 70° C.). These temperatures are well belowtraditional integrated circuit and semiconductor processingtemperatures, which enable the use of any of a variety of relativelyinexpensive supports, such as flexible polymeric supports. Thus,embodiments of the invention enable production of relatively inexpensivedevices on flexible substrates without the need for photolithography andenable rapid pattern changes due to printing the patterns.

The substrates used in the present invention can be any material thatacts as a mechanical support for the subsequently coated layers. Thesubstrate can include a rigid material such as glass, silicon, ormetals. Particularly useful metals include stainless steel, steel,aluminum, nickel, and molybdenum. The substrate can also include aflexible material such as a polymer film or paper. Useful substratematerials include organic or inorganic materials. For example, thesubstrate can include inorganic glasses, ceramic foils, polymericmaterials, filled polymeric materials, coated metallic foils, acrylics,epoxies, polyamides, polycarbonates, polyimides, polyketones,poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene)(sometimes referred to as poly(ether ether ketone) or PEEK),polynorbornenes, polyphenyleneoxides, poly(ethylenenaphthalenedicarboxylate) (PEN), poly(ethylene terephthalate) (PET),poly(ether sulfone) (PES), poly(phenylene sulfide) (PPS), andfiber-reinforced plastics (FRP). The thickness of substrate 110 canvary, typically from about 100 μm to about 1 cm.

A flexible support or substrate can be used in the present invention.Using a flexible substrate allows for roll processing, which can becontinuous, providing economy of scale and economy of manufacturingrelative to flat or rigid supports. The flexible support chosen ispreferably capable of wrapping around the circumference of a cylinder ofless than about 50 cm in diameter, more preferably 25 cm in diameter,and most preferably 10 cm in diameter, without distorting or breaking,using low force as by unaided hands. The preferred flexible support canbe rolled upon itself. Additional examples of flexible substratesinclude thin metal foils such as stainless steel provided the foils arecoated with an electrically insulating material layer to electricallyisolate any electric components such as thin film transistors. Nominallyrigid materials that are flexible due to their thinness may also beused. These include glass at thicknesses below 200 μm and metals atthicknesses below 500 μm.

In some example embodiments, the substrate can include a temporarysupport or support material layer, for example, when additionalstructural support is desired for a temporary purpose, e.g.,manufacturing, transport, testing, or storage. In these exampleembodiments, substrate can be detachably adhered or mechanically affixedto the temporary support. For example, a flexible polymeric support canbe temporarily adhered to a rigid glass support to provide addedstructural rigidity during the transistor manufacturing process. Theglass support can be removed from the flexible polymeric support aftercompletion of the manufacturing process.

The substrate can be bare indicating that it contains no substantialmaterials on its surface other the material from which it is composed.The substrate can include various layers on the surface. These layersinclude subbing layers, adhesion layers, release layers, wetting layers,hydrophilic layers, and hydrophobic layers. The substrate surface can betreated in order to promote various properties. These treatments includeplasma treatments, corona discharge treatments, and chemical treatments.

The substrate can also include on its surface patterned materials. Thesepatterns can include patterns that modulate light transmission orelectrical conductivity within or on the substrate. The patterns caninclude complete devices, circuits, or active elements existing on thesubstrate. The patterns can include portions of devices, circuits, oractive elements awaiting subsequent processing steps for completion.

The thin-film transistors of the present invention are composed ofdielectric, semiconductor and conductor materials. In preferredembodiments of the present invention the dielectric, semiconductor andconductor materials are inorganic thin films. A dielectric material isany material that is a poor conductor of electricity. Such materialstypically exhibit a bulk resistivity greater than 10¹⁰ Ω-cm. Examples ofdielectrics are SiO₂, HfO, ZrO, Si_(x)N_(y), and Al₂O₃. A semiconductoris a material in which electrical charges can move but in which theconcentration of electrical charges can be substantially modulated byexternal factors such as electrical fields, temperature, or injection ofelectrical charges from a neighboring material. Examples ofsemiconductors include silicon, germanium, and gallium arsenide.Particularly preferred semiconductors are zinc oxide-basedsemiconductors including, zinc oxide, indium zinc oxide, and galliumindium zinc oxide. The semiconductors can be doped to render them n-typeor p-type, or to modulate the number of charge carriers present.Conductors of the present invention include metals, such as Al, Ag, Au,Cr, Mo, or In and inorganic conducting oxides, such as indium doped tinoxide (ITO) or aluminum-doped zinc oxide (AZO).

The dielectric and semiconductor inorganic materials layers of thepresent invention are conformal, and are preferably deposited using anatomic layer deposition (ALD) process. ALD is a process which is used toproduce coatings with thicknesses that can be considered consistent,uniform, or even exact. ALD produces coatings that can be consideredconformal or even highly conformal material layers. Generally described,an ALD process accomplishes substrate coating by alternating between twoor more reactive materials commonly referred to as precursors, in avacuum chamber. A first precursor is applied to react with thesubstrate. The excess of the first precursor is removed from the vacuumchamber. A second precursor is then applied to react with the firstprecursor on the substrate. The excess of the second precursor isremoved from the vacuum chamber and the process is repeated.

Recently, a new ALD process called spatial atomic layer deposition hasbeen developed which negates the need for a vacuum chamber. Thisprocess, commonly referred to as S-ALD or SALD, is described in at leastone of U.S. Pat. No. 7,413,982, U.S. Pat. No. 7,456,429, U.S. Pat. No.7,789,961, and US 2009/0130858, the disclosures of which areincorporated by reference herein. SALD produces coatings withthicknesses that can be considered consistent, uniform, or even exact.SALD produces coatings that can be considered conformal or even highlyconformal material layers. SALD is also compatible with a lowtemperature coating environment. Additionally, SALD is compatible withweb coating, making it attractive for large scale production operations.Even though some web coating operations may experience alignment issues,for example, web tracking or stretching issues, the architecture of thepresent invention reduces reliance on high resolution or very finealignment features during the manufacturing process. As such, SALD iswell suited for manufacturing the present invention.

The preferred process of the present invention employs SALD, acontinuous spatially dependent ALD (as opposed to pulsed or timedependent ALD). The process of the present invention allows operation atatmospheric or near-atmospheric pressures and is capable of operating inan unsealed or open-air environment. The process of the presentinvention is adapted such that material is deposited only in selectedareas of a substrate.

Atomic layer deposition can be used in embodiments of the presentinvention to deposit a variety of inorganic thin films that are metalsor that comprise a metal-containing compound. Such metal-containingcompounds include, for example (with respect to the Periodic Table) aGroup V or Group VI anion. Such metal-containing compounds can, forexample, include oxides, nitrides, sulfides or phosphides of zinc,aluminum, titanium, hafnium, zirconium or indium, or combinationsthereof.

Oxides that can be made using the process of the present inventioninclude, but are not limited to: zinc oxide (ZnO), aluminum oxide(Al₂O₃), hafnium oxide, zirconium oxide, indium oxide, tin oxide, andthe like. Mixed structure oxides that can be made using the process ofthe present invention can include, for example, InZnO. Doped materialsthat can be made using the process of the present invention can include,for example, ZnO:Al, Mg_(x)Zn_(1-x)O, and LiZnO.

Metals that can be made using the process of the present inventioninclude, but are not limited to: copper, tungsten, aluminum, nickel,ruthenium, and rhodium. It will be apparent to the skilled artisan thatalloys of two, three, or more metals can be deposited, compounds may bedeposited with two, three, or more constituents, and such things asgraded films and nano-laminates can be produced as well.

The embodiments of the present invention all relate to thin filminorganic materials and devices that contain them. Example embodimentsof the present invention use selective area deposition (SAD) incombination with atomic layer deposition (ALD). SAD employs a patternedmaterial referred to as a “deposition inhibitor material”, “depositioninhibiting material”, or simply an “inhibitor” that inhibits the growthof a thin film material on the substrate when the substrate is subjectedto an atomic layer deposition. There have been previous studies on usinga selective area to generally pattern ALD coatings and more specificallyto pattern a semiconductor layer. For example, Sinha et al. (J. Vac.Sci. Technol. B 24 6 2523-2532 (2006)), have remarked that selectivearea ALD requires that designated areas of a surface be masked or“protected” to prevent ALD reactions in those selected areas, thusensuring that the ALD film nucleates and grows only on the desiredunmasked regions. It is also possible to have SAD processes where theselected areas of the surface area are “activated” or surface modifiedin such a way that the film is deposited only on the activated areas.There are many potential advantages to selective area depositiontechniques, such as eliminating an etch process for film patterning,reduction in the number of cleaning steps required, and patterning ofmaterials which are difficult to etch. The use of SAD to patternmultilayer dielectric layers for use in electronic devices has beenreported in at least one of U.S. Publication No. 2014/0065838, USPublication No. 2014/0061869, U.S. Pat. No. 8,927,434, U.S. Pat. No.8,791,023, U.S. Pat. No. 8,846,545, US Publication No. 2014/0061795, USPublication No. 2014/0061648 and U.S. Pat. No. 8,653,516, thedisclosures of which are incorporated by reference herein.

By inhibiting the growth where the deposition material is present, thedeposition only deposits in regions (selective areas) of the substratewhere the inhibitor is not present. The phrase “deposition inhibitormaterial” and its equivalents refer herein to any material on thesubstrate that inhibits the deposition of material during atomic layerdeposition (ALD). The “deposition inhibitor material” includes thematerial applied to the substrate as well as the material resulting fromany optionally subsequent crosslinking or other reaction that modifiesthe material that may occur prior to depositing an inorganic thin filmon the substrate by atomic layer deposition. A polymeric depositioninhibitor material may be crosslinked after applying the polymer ontothe substrate, before or during the patterning step.

The deposition inhibitor material can be a compound or polymer that,after being applied, is subsequently polymerized, crosslinked, orpolymerized and crosslinked. The deposition inhibitor material can be acompound or polymer that forms a self-assembled monolayer on thesubstrate. Polymers are preferably addition polymers such as, forexample, a poly(perfluoroalkyl methacrylate); poly(methyl methacrylate);poly(cyclohexyl methacrylate); poly(benzyl methacrylate);poly(iso-butylene); poly(9,9-dioctylfluorenyl-2,7-diyl); polystyrene;poly(vinyl alcohol); poly(hexafluorobutyl methacrylate), and copolymersthereof, wherein the alkyl has one to six carbon atoms.

Crosslinking can be used to insolubilize a polymeric depositioninhibitor material after application onto the surface of the substrate.The crosslinking can occur prior to patterning or can occur duringpatterning in order to contribute to the patterning step, for example,by employing crosslinking initiated by, and patterned by, actinicradiation, followed by removal of non-crosslinked polymer, for example,by solvent.

The deposition inhibiting material layer includes one of aself-assembled monolayer, a polymer, and a water-soluble polymer. Theself-assembled monolayer can be performed by exposing the substrate to avapor, a liquid, or a liquid solution of a precursor material. Thepolymer can be soluble in any convenient solvent and can have any usefulmolecular weight, preferably in the range of 2,000 to 2,000,000. It caninclude a single functional group, or may include a plurality offunctional groups. In the case of a plurality of functional groups, thepolymer can be a random, periodic, or block polymer. Polymers soluble inpolar solvents such as water, alcohols, or ketones are particularlypreferred. Polymers can include amide groups, such as poly(amide),poly(vinylpyrollidone), and poly(2-ethyl-oxazoline). Polymers mayinclude ether linkages, such as poly(ethylene glycol). Polymers caninclude alcohol functionalities, such as poly(vinyl alcohol). Polymerscan include neutralized acid groups such as sodium poly(styrenesulfonate) and the sodium salt of poly(acrylic acid).

In some embodiments, the deposition inhibitor material is chosenspecifically for the material to be deposited. The deposition inhibitormaterial has a given inhibition power. The inhibition power is definedas the layer thickness at or below which the deposition inhibitormaterial is effective. Preferably, the deposition inhibitor material,during use, exhibits an inhibition power of at least 50 Å, morepreferably at least 100 Å, most preferably at least 300 Å. Thedeposition of the deposition inhibitor material can be in a patternedmanner, such as using inkjet, flexography, gravure printing,microcontact printing, offset lithography, patch coating, screenprinting, or transfer from a donor sheet. In alternative embodiments, auniform layer of the deposition inhibitor material can be deposited andthen patterned to form a patterned layer of the deposition inhibitormaterial. Preprocessing treatments for patterning the inhibitor includepatterning of substrate prior to inhibitor application to modify thehydrophobilicity, electric charge, absorption, or roughness of thesubstrate. Post processing treatments include light exposure, lightexposure and subsequent liquid based development, and ablation.

Providing the patterned deposition inhibiting material layer on thesubstrate includes using at least one of an inkjet printing process, aflexographic printing process, a gravure printing process, and aphotolithographic printing process. The active inhibiting material canbe suspended or dissolved in a solvent or vehicle. The material caninclude surfactants, stabilizers, or viscosity modifiers. The printedmaterial can be dried using natural convection, forced convection, orradiant heat. The material can be treated to change its morphology orchemical composition. A preferred chemical composition change is tocrosslink the material. The change in morphology or chemical compositioncan be accomplished by exposure to a vapor phase or liquid phasereactant, or treatment with heat or light. Preferred processes includethe crosslinking of material with ultra-violet (UV) light.

In some embodiments, the deposition inhibitor material can be used inthe reentrant profile of the vertical-support-element to prevent thegrowth thin-film material inside the reentrant profile. Depositioninhibitor in the reentrant profile can be used to pattern a conformalconductive layer, such as a conductive metal oxide, the resulting in twovertically spaced apart electrodes. U.S. application Ser. Nos.14/198,621; 14/198,623; 14/198,636; and 14/198,643, filed Mar. 6, 2014,describe the formation of vertical transistors using selective areadeposition to define the source and drain, and are hereby incorporatedby reference. As described, the deposition inhibitor can be patternedvia exposure (photo-lithographically), or by printing. When thedeposition inhibitor is applied via printing, patterns can be designedsuch that the inhibitor wicks (moves by capillary action) along thereentrant profile of the vertical-support-element. Previouslyincorporated U.S. application Ser. No. 14/198,628 filed Mar. 6, 2014describes the use selective area deposition to form vertically separatedelectrodes over an insulating vertical-support-element formed from astructural polymer post and an inorganic cap.

According to one embodiment of the present invention, an electronicdevice comprises a substrate and a vertical-support-element on thesubstrate. The vertical-support-element extends away from the substrateto a top and includes a first edge having a first reentrant profile.There is a conformal semiconductor layer in contact with thevertical-support-element in the reentrant profile, a first electrodelocated in contact with a first portion of the semiconductor layer overthe top of the vertical-support-element, and a second electrode locatedin contact with a second portion of the semiconductor layer over thesubstrate and not over the vertical-support-element, and adjacent to thefirst edge of the vertical-support-element.

There is a conformal insulating material layer on the semiconductorlayer in the reentrant profile, and a conformal conductive gate on theconformal insulating material layer over the first edge of thevertical-support-element in the first reentrant profile. The firstelectrode and the second electrode define a transistor having a channelin the semiconductor layer between the first electrode and the secondelectrode.

Turning now to the Figures, a schematic cross-sectional view of verticaltransistors 100 and 200 of the present invention is shown FIG. 1a ,taken along the line A-A′ of the plan view shown in FIG. 1b . Theelectronic device shown in FIG. 1a , has TFTs 100 and 200 on a substrate110, where each TFT 100, 200 is a vertical transistor, where thevertical portion is defined by a vertical-support-element 120 (alsosometimes called a post structure herein). The vertical-support-element120 is on the substrate 110, and the vertical-support-element extendsaway from the substrate 110 to a top, the vertical-support-elementincluding a first edge having a first reentrant profile 140. There is aconformal semiconductor layer 150 in contact with thevertical-support-element 120 in the reentrant profile 140, a firstelectrode 180 located in contact with a first portion of thesemiconductor layer 150 over the top of the vertical-support-element 120and a second electrode 170 located in contact with a second portion ofthe semiconductor layer 150 over the substrate 110 and not over thevertical-support-element 120, and adjacent to the first edge of thevertical-support-element. There is a dielectric layer 130 on thesemiconductor layer 150 in the reentrant profile 140, and a conformalconductive gate 125 on the conformal insulating material layer 130 overthe first edge of the vertical-support-element in the first reentrantprofile 140. The first electrode 180 and the second electrode 170 definethe vertical transistor 100 having a channel in the semiconductor layer150 between the first electrode 180 and the second electrode 170.

Vertical transistors 100 and 200 are formed over a single poststructure, and are connected in series, and as shown share a commongate. For simplicity, the following description will focus on top-gatevertical transistor 100 with the understanding that the descriptionsapply equally to top-gate vertical transistor 200. Top-gate verticaltransistor 100 includes a substrate 110. Substrate 110, often referredto as a support, can be rigid or flexible as described above. Onsubstrate 110 is a vertical-support-element 120, thevertical-support-element 120 having a height dimension extending awayfrom the substrate 110 to a top defined by a length dimension, and awidth dimension, over the substrate 110, the vertical-support-elementhaving edges along the height dimension. The top of thevertical-support-element 120, extends beyond the edges of thevertical-support-element 120 in at least the width dimension. As shownin FIG. 1a , the portion of the top of the vertical-support-element 120which extends beyond the edges of the vertical-support-element 120 formsa first reentrant profile 140 and a second reentrant profile 145.

To better understand the requirements for the vertical-support-element120, we will turn briefly to FIGS. 3a through 3c and FIGS. 4a through 4f. As shown in FIGS. 3a, 3b and 3c the vertical-support-element 120 canhave any shape which has a height dimension 530 a, 530 b, 530 cextending away from the substrate 110 to a top 535 a, 535 b, 535 c. Thevertical-support-element 120 also has edges along the height dimension,a first edge 510 a, 510 b, 510 c including a first reentrant profile 540a, 540 b, 540 c and a second edge 520 a, 520 b, 520 c opposite the firstedge 510 a, 510 b, 510 c and including a second reentrant profile 545 a,545 b, 545 c. The vertical-support-element 120 has a width at the top500 a, 500 b, 500 c, and a minimum width 505 a, 505 b, 505 c between thetop and the substrate that is smaller than the width of the top. Thereentrant profile is defined as the profile of thevertical-support-element 120 that lies within the boundaries of the topof the vertical-support-element 120, the substrate 110, and a line drawnorthogonal to the substrate from the edge of the top (as shown by linesP-P′ in FIGS. 3a, 3b and 3c ). To aid in understanding, the firstreentrant profile 340 a, 340 b, and 340 c in each figure is shown inbold. Additionally, the vertical-support-element 120 can have portionsthat extend beyond the reentrant profile, as shown in FIG. 3c . Thevertical-support-element 120 shown in these figures can be fully formedof insulating materials. In alternative embodiments for use inarchitectures requiring a bottom-gate, the vertical-support-element canbe fully formed of conducting materials. FIGS. 3a, 3b and 3c illustratereentrant profiles obtainable with either an electrically conductivecore or insulating structural polymer core. It should be recognized thatthe first and second reentrant profiles shown in FIGS. 3a, 3b and 3c(540 a, 540 b, 540 c and 545 a, 545 b, and 545 c) are equivalent to thefirst and second reentrant profiles 140 and 145 of FIGS. 1a and 1 b.

In some embodiments, the vertical-support-element 120 can be formed froma post and cap, preferably a structural polymer post and an inorganiccap. The range of useful variants of the reentrant profile formed with apost 20 and cap 30 can be better understood from FIGS. 4a through 4fwhich each contain a closer cross-sectional view of reentrant profile140 of a vertical-support-element 120 that has been formed from a post20 and cap 30. As shown, reentrant profile 140 can have any shape thatas defined by the extension of a cap 30 beyond the edge of post 20.Preferably, the cap extends beyond the edge of the post by a distancethat is less than the eight of the post. The reentrant profile 140 isdefined as the profile that lies within the boundaries of the cap 30,the substrate 110 and a line drawn orthogonal to the substrate from theedge of the cap at the point of furthest extension (as shown by linesP-P′ in FIGS. 4a through 4f ). This is equivalent to the requirementillustrated in FIGS. 3a through 3d that the top of thevertical-support-element 120 extends beyond the base of thevertical-support-element. To aid in understanding, the first reentrantprofile 140 in each figure is shown in bold. As shown, each reentrantprofile has an associated depth, d, which is defined as the maximumextension of the cap from the post, measured parallel to the substrate.FIG. 4a illustrates a simplified profile, consistent withvertical-support-element 120 profile that is used to demonstrate theembodiments of the present invention. As shown in FIGS. 4d and 4f , thecap 30 can have a non-uniform edge profile. FIGS. 4b, 4c, and 4f are allembodiments where the edges of the post are not straight vertical edgesorthogonal to the substrate; instead they can have any profile as longas the edge does not extend beyond the end of the cap.

Returning now to FIGS. 1a and 1b , there are three electrodes(180,170,175) that are the source and drain electrodes for top-gate VTFT100 and 200. Source and drain have conventionally accepted meanings. Fortop-gate VTFT 100, either the first electrode 180 or the secondelectrode 170 can be designated the source (or drain) as is required bythe application or circuit. The first electrode 180 and the secondelectrode 170 can include a conductive layer stack. The first electrode180, second electrode 170 and third electrode 175 can be a singleconductive material, as shown in FIG. 1a , or can comprise any number ofconductive material layers. The first electrode 180 is located incontact with a first portion of the semiconductor layer 150 over the topof the vertical-support-element 120. The second electrode 170 electrodeis located in contact with a second portion of the semiconductor layer150 over the substrate 110 and not over the vertical-support-element120. As shown in FIG. 1a , the first electrode can also be in contactwith the top of the vertical-support-element. The first electrode 180and second electrode 170 define a first semiconductor channel oftop-gate VTFT 100, where semiconductor channel has the conventionallyaccepted meaning, and is in the semiconductor layer 150 between thefirst electrode 180 and second electrode 170.

The conformal semiconductor layer 150 conforms to the first reentrantprofile 140 and second reentrant profile 145 of transistors 100 and 200and maintains the shape, and is in contact with thevertical-support-element 120 in the reentrant profile 140. Semiconductormaterial layer 150 is a conformal semiconductor material layer.Preferably the semiconductor layer 150 is a thin film inorganicsemiconductor material layer. The conformal dielectric layer 130conforms to the reentrant profiles 140 and 145, and is in contact withthe semiconductor layer 150. Dielectric layer 130 can be said to coatand maintain the reentrant profiles 140 and 145. The dielectric layer130 is a conformal insulating material layer. Dielectric layer 130 isoften referred to as a dielectric material layer, or an insulatinglayer, and can be formed of a single material layer or multipledielectric material layers (multi-layer dielectric stack). Preferablythe dielectric layer 130 is a thin film inorganic dielectric materiallayer.

The conductive conformal gate layer 125 is on the conformal dielectriclayer 130 in the first reentrant profile 140. The conductive conformalgate layer 125 is on the side of the semiconductor layer 150 oppositethe vertical-support-element 120, and can be said to be a top-gate. Theconductive conformal gate layer 125 can be a single conductive material,as shown in FIG. 1a , or can comprise any number of conductive materiallayers. The conductive conformal gate layer is preferably a conductivemetal oxide material.

As shown in FIG. 1a , the vertical TFT 100 is formed in series andshares a common gate with vertical TFT 200. In this configuration thereare three electrodes including the shared first electrode 180 over thevertical-support-element 120, a second electrode 170 for TFT 100 notover the vertical-support-element 120, and a third electrode 175 for TFT200 not over the vertical-support-element 120 (on the opposite side ofvertical-support-element 120 from the second electrode 170 electrode ofTFT 100). As shown, the vertical-support-element 120 has a second edgealong the height dimension and the top of the vertical-support-elementextends beyond the second edge to define a second reentrant profile 145.The third electrode 175 is located in contact with a third portion ofthe semiconductor layer 150 over the substrate 110 and not over thevertical-support-element 120, and adjacent to the second reentrantprofile 145. The distance between the first electrode 180 and thirdelectrode 175 is greater than zero when measured orthogonal to thesubstrate surface. The first electrode 180 and the third electrode 175define a semiconductor channel of the second transistor 200.

The first electrode 180 and the second electrode 170 define the channelof the first VTFT 100, and the third electrode 175 and the firstelectrode 180 define the channel of the second VTFT 200. Theconfiguration shown in FIGS. 1a and 1b allows contact to the verticaltransistors 100 and 200 to be made on the substrate level so thetransistors are operated in series, rather than making connection at thetop of the vertical-support-element 120. It should be understood thatthe present invention includes a single vertical transistor where theelectrodes are disposed as 170 and 180 in FIG. 1 a.

As shown, the electrically conductive gate layer 125 functions as thegate for transistors 100 and 200. In some example embodiments oftransistor 100, electrode 170 functions as the drain of transistor 100and electrode 180 functions as the source of transistor 100. In otherexample embodiments of transistor 100, electrode 170 functions as thesource and electrode 180 functions as the drain. The semiconductordevice is actuated in the following manner. After transistor 100 isprovided, a voltage is applied between the electrode 170 and theelectrode 180. A voltage is also applied to the electrically conductivegate 125 to electrically connect the electrode 170 and the electrode180.

Still referring to FIGS. 1a and 1b , vertical transistor 200 is formedat the same time as vertical transistor 100 is formed. Transistor 200can be actuated in the following manner. A voltage is applied betweenthe electrode 175 and the electrode 180, which is shared with verticaltransistor 100. A voltage is applied to the gate layer 125, which isshared with vertical transistor 100, to electrically connect theelectrode 175 and 180.

Alternatively, transistor 100 and transistor 200 can be actuated inseries by applying a voltage between electrode 170 and electrode 175. Avoltage is applied to the gate layer 125, which simultaneouslyelectrically connects electrode 180 to electrode 170 and connectselectrode 180 to electrode 175. This can be advantageous for circuitapplications because external electrical connections do not need to bemade to the elevated third electrode 180.

In other embodiments, the conductive layer can be patterned toindependently gate transistor 100 and 200; in these embodiments gates125 and 127 can be formed as shown in FIGS. 6a and 6b each being aseparate region within a common conductive material layer.

The reentrant profile 140 of transistor 100 allows a dimension of thesemiconductor channel of the transistor 100 to be associated with thethickness of the vertical-support-element 120, which is defined by theheight of the vertical-support-element 120. Advantageously, thisarchitecture of the present invention reduces reliance on highresolution or very fine alignment features during the manufacture oftransistors that include short channels. Furthermore, the separation ofthe first electrode 180 and second electrode 170 is primarily determinedby the reentrant profile 140 in the vertical-support-element 120.Additionally, the first electrode 180, second electrode 170 and thethird electrode 175 are formed simultaneously and have the same materialcomposition and layer thickness.

As shown in FIGS. 1a and 1b , the second electrode 170 and the thirdelectrode 175 are located adjacent to the first and second reentrantprofiles 140, 145, respectively. The second electrode 170 and the thirdelectrode 175 are vertically spaced from the first electrode 180 due tothe height of the vertical-support-element 120. The second electrode 170and the first electrode 180 define a first channel having ends of thefirst transistor 100 and the third electrode 175 and the first electrode180 define a second channel having ends of the second transistor 200.Stated another way, the distance between the first electrode 180 andsecond electrode 170 is greater than zero when measured orthogonal tothe substrate surface. Together with the gate 125, dielectric layer 130,and semiconductor layer 150 define the first top-gate verticaltransistor 100, including a portion of the channel which is verticalwith respect to the substrate surface.

The transistor architecture shown in FIGS. 1a and 1b can be formed fromvarious materials. The first electrode 180, second electrode 170, andthird electrode 175 can be any conductive material, and in someembodiments are transparent conductive oxides. It is one advantage ofthe present invention that the vertical transistors can be fullytransparent. It is also an advantage that all of the materials can bemetal oxides that are deposited from a common piece of equipment. Theinorganic material cap, the conformal conductive material gate layer,the conformal insulating material layer, the conformal semiconductormaterial layer, the first electrode, the second electrode, and the thirdelectrode can each include a metal oxide. The vertical-support-element120 can comprise a polyester, polyetherester, polyamide, polyesteramide,polyurethane, polyimide, polyetherimide, polyurea, polyamideimide,polyphenyleneoxide, phenoxy resin, epoxy resin, polyolefin,polyacrylate, polyethylene-co-vinyl alcohol, or a copolymer thereof, ora mixture thereof; preferably the vertical-support-element 120 comprisesan epoxy resin or polyimide.

The vertical-support-element 120 can be used for various verticaltransistor geometries, depending on the processing tools available. Theschematic cross-sectional view of vertical top-gate transistors 102 and202 of an embodiment of the present invention is shown FIG. 2a . Thevertical transistors 102 and 202 are identical in components andoperation to the vertical transistors 100 and 200 shown in FIG. 1a . Theembodiment shown in FIG. 2a is representative of vertical transistors102 and 202 formed by a line-of-sight deposition process for the firstelectrode 180, the second electrode 170 and the third electrode 175.Line-of-sight deposition processes include evaporation and sputtering.As shown, the first electrode 180 and the second electrode 170 arealigned near vertically. The first electrode 180 has an end (C2) and thesecond electrode 170 has an end (C2′), such that the end (C2) of thefirst electrode 180 and the end (C2′) of the second electrode 170 arevertically aligned.

An alternative embodiment of the present invention is shown in theschematic cross-sectional view of vertical transistors 103 and 203 inFIG. 2b . The vertical transistors 103 and 203 are identical incomponents and operation to the vertical transistors 100 and 200 shownin FIG. 1a . The embodiment shown in FIG. 2b is representative ofvertical transistors 103 and 203 formed by a selective area deposition(SAD) process where the first reentrant profile 140 and second reentrantprofile 145 are filled with a deposition inhibitor by capillary action,and the first electrode 180, the second electrode 170 and the thirdelectrode 175 are deposited using ALD. As shown, the first electrode 180and the second electrode 170 are spaced further apart resulting in alonger channel length of the first transistor 103 than seen in theprevious embodiments shown in FIG. 1a by transistor 100 and in FIG. 2aby transistor 102. As shown, a SAD process results in verticaltransistors where a portion of the channel is vertically oriented withrespect to the substrate as with the previous embodiments. As shown,vertical top-gate transistors 103 and 203 each have a portion of theirchannel that is parallel to the substrate surface in addition to theportion which is not parallel.

The wicking process used to form the vertical transistor of FIG. 2btypically results in the inhibitor wetting out of the reentrant profileonto the substrate. The channel defined by the first electrode 180 andthe second electrode 170 preferably has a length dimension that is lessthan 10 times the height of the vertical-support-element 120. The use ofa wicked deposition inhibitor also results in a transistor channel thathas a length that varies along the width dimension, where the transistorwidth is into the page and the length is defined along the reentrantprofile 140. The distance between C1 and C1′ will vary along the widthof the channel which is along the length of the vertical-support-element(into the page dimension) of the transistor 103. The channel defined bythe first electrode 180 and the second electrode 175 has a widthdimension and a length dimension, and the length dimension can varyalong the width dimension of the transistor 103.

As shown in FIG. 2b , the second electrode 170 and the third electrode175 are located adjacent to the first and second reentrant profiles 140,145, respectively. A first line extending between the ends of the firstchannel is shown in FIG. 2b by line C1-C1′. A second line extendingbetween the ends of the second channel is shown in FIG. 2b by lineC3-C3′. As shown in FIG. 2b the first line C1-C1′ is not parallel to thesecond line C3-C3′. Vertical transistors having this relationshipbetween their channels cannot be formed over a broad areas byline-of-sight deposition techniques such as thermal evaporation ofmetals. In alternative embodiments, the lines can be diverging orconverging.

FIGS. 2c and 2d are schematic cross-sectional views of an alternativeembodiment of the present invention. In both FIGS. 2c and 2d the sourceand drain of VTFT 104, 105 electrodes make contact to the opposite sideof the semiconductor layer than was shown in FIG. 1a , and have atop-gate coplanar geometry. As seen in FIG. 2c , the first electrode 180is over the vertical-support-element 120 and in contact with thesemiconductor layer 150. The second electrode 170 is adjacent to thefirst reentrant profile 140, and in contact with a second portion of thesemiconductor layer 150 located over the substrate 110 and not over thevertical-support-element 120. The position of the electrodes and thesemiconductor layer are a consequence of the fabrication process. Asshown in FIG. 2c , the conformal semiconductor layer 150 was depositedprior to depositing the first and second electrodes 180, 170, and is incontact with the vertical-support-element 120.

The alternative embodiment shown in FIG. 2d illustrates the optionalsecond conformal dielectric layer 115 covering thevertical-support-element 120 and at least a portion of the substrate110. The conformal dielectric layer 115 is located at least between thesemiconductor layer 150 and the vertical-support-element 120. As shownin FIG. 2d , TFTs 105 and 205 are a vertical transistors, where thevertical portion is defined by a vertical-support-element 120, which iscovered by a conformal dielectric material layer 115. With the exceptionof the conformal dielectric material layer 115, the elements of thevertical thin film transistor 105 and 205 shown in FIG. 2d are the sameas those for vertical thin film transistors 104 and 204 and should beunderstood from the description of FIG. 2c . The dielectric materiallayer 115 in this embodiment can be used to encapsulate the polymer postand cap used to form the vertical-support-element 120, and can provide asingle material surface for building the vertical transistor of thepresent invention. The addition of dielectric material layer 115 can beused to avoid issues of non-uniform nucleation or thin film growth onthe vertical-support-element 120 and substrate surfaces 110. Preferablythe dielectric material layer 115 is an inorganic thin film dielectricmaterial layer. The dielectric material layer 115 is a conformal layer,which is preferably deposited using ALD due to the conformal nature ofthe ALD process. As shown, the dielectric material layer 115 maintainsthe first and second reentrant profiles 140 and 145.

An alternative embodiment of the present invention is illustrated by theschematic cross-sectional view of vertical transistors 106 and 206 shownFIG. 5a , taken along the line A-A′ of the plan view shown in FIG. 5b .This embodiment of the present invention has reduced overlapcapacitance. As shown, the gate layer 125 is composed of two sections125 a and 125 b which are in the reentrant profiles 140,145 defined bythe vertical-support-element 120, but not over the top of thevertical-support-element 120. In this embodiment the two sections 125 aand 125 b are each confined to be within the dimensions of the first andsecond reentrant profiles 140 and 145, and do not extend beyond thedimensions of the top of the vertical-support-element 120. Thus theconformal conductive material gate layer 125 is only located in thefirst reentrant profile 140 and in the second reentrant profile 145 inthe region of the channels. The two sections of the gate layer 125 canbe connected to each other outside of the of the channel region, asillustrated in FIG. 5b . As shown, the two portions 125 a and 125 b ofgate layer 125 physically and electrically connected forming a commongate for the vertical top-gate transistors 106, 206 that are in series,and are a variation of the embodiment shown in FIGS. 1a and 1b . Thevertical transistors 106,206 having the gate layer 125 of FIGS. 5a and5b , with two portions 125 a and 125 b, functions in the same manner asthe vertical transistors 100 and 200 of FIGS. 1a and 1b . In alternativeembodiments, the two portions of the gate can be patterned such thateach TFT has an independently operable gate.

The schematic cross-sectional view of vertical transistors 107 and 207shown FIG. 6a , taken along the line A-A′ of the plan view shown in FIG.6b is an embodiment of the present invention which results in twoseparate vertical top-gate transistors on opposite sides of thevertical-support-element 120, separated along the width of thevertical-support-element 120. In this embodiment the gate layer ispatterned to be discontinuous, resulting in two separate patternedconductive conformal gates 125 and 127 for VTFTs 107 and 207respectively. Similar to FIGS. 1a and 1b , the second electrode 170 andthird electrode 175 are located adjacent to the first and secondreentrant profiles 140, 145 respectively, on either side ofvertical-support-element 120 and not over the top of thevertical-support-element 120. The first electrode 180 is over thevertical-support-element 120, and adjacent to the first reentrantprofile 140. The first electrode 180 and the second electrode 170 definethe semiconductor channel of the first VTFT 107. The fourth electrode185 is over the vertical-support-element 120, and adjacent to the secondreentrant profile 145. The fourth electrode 185 and the third electrode175 define the semiconductor channel of the second VTFT 207. The secondelectrode 180 and the fourth electrode 185 are physically andelectrically separate, such that the two separate vertical transistors107 and 207 are independently operable and formed using a singlevertical-support-element 120. As shown in FIGS. 6a and 6b , thesemiconductor layer can be common, but patterned into two separateregions 150 and 155. In alternative embodiments, VTFT 107 and VTFT 207can be formed with different semiconductor materials.

The schematic cross-sectional view of vertical top-gate transistor 108is shown FIG. 7a , taken along the line A-A′ of the plan view shown inFIG. 7b is an alternative embodiment of the present invention where asingle VTFT is formed over the vertical-support-element 120. As shown inFIGS. 7a and 7b is a single vertical top-gate transistor 108 formed withreentrant profile 140. In this embodiment the gate layer is patterned tobe in first reentrant profile 140, but not in the second reentrantprofile 145. The source/drain electrode configuration for VTFT 108 issimilar to VTFT 100, wherein the first electrode 181 is in contact withthe semiconductor 150 over the top of the vertical-support-element 120.The second electrode 170 is located adjacent to the first reentrantprofiles 140 and not over the top of the vertical-support-element 120.The first electrode 181 is over the vertical-support-element 120, andpatterned such that it conformally coats the second reentrant profile145. The first electrode 181 and the second electrode 170 define thesemiconductor channel of the top-gate VTFT 108.

Bottom-gate transistors can also be formed over thevertical-support-element 120. Previously incorporated U.S. applicationSer. No. 14/198,628, filed Mar. 6, 2014, describes bottom-gate verticaltransistors formed over a polymer post with an inorganic cap. Thecombination of the polymer post and inorganic cap can be used to formthe vertical-support-element 120 of the present invention, and should beunderstood from the previous description. FIG. 8a is a schematiccross-sectional view of bottom-gate vertical transistors 801 and 802,taken along the line A-A′ of the plan view shown in FIG. 8b . As shownin FIG. 8a , TFTs 801 and 802 each have a vertical transistor structure,where the vertical portion is defined by the vertical-support-element120. The gate layer 825 is in contact with at least the edge of thevertical-support-element 120, the dielectric layer 830 is in contactwith the gate 825, and the semiconductor layer 850 is in contact withthe first electrode 880. The vertical-support-element 120 and gate layer825 form an electrically conductive gate structure 820 having a firstreentrant profile 140 and a second reentrant profile 145.

Vertical transistors 801 and 802 have a common gate 825 and areconnected in series and are formed over a singlevertical-support-element 120. Vertical transistors 801 and 802 are thebottom-gate analogs of the top-gate vertical transistors 100 and 200shown in FIGS. 1a and 1b . For simplicity, the following descriptionwill focus on vertical transistor 801 with the understanding that thedescriptions apply equally to vertical transistor 802. Verticaltransistor 801 includes a substrate 110 and a vertical-support-element120, which should be understood from the previous descriptions. Aconformal conductive layer forms the gate layer 825 on the edges of thevertical-support-element 120 in the reentrant profile and over at leasta portion of the substrate 110 not over the VTFT structure 120. As shownin FIG. 8a , the gate layer 825 conformally covers the top and reentrantprofiles 140, 145 of the vertical-support-element 120, and is also incontact with the substrate 110. The insulating layer 830 conforms to thereentrant profiles 140 and 145 of transistors 801 and 802. Insulatinglayer 830 can be said to coat and maintain the reentrant profiles 140and 145, and is in contact with the gate layer 825. Dielectric layer 830is a conformal insulating material layer. Semiconductor material layer850 also conforms to the first reentrant profile 140 and secondreentrant profile 145 of transistors 801 and 802 and maintains theshape, and is in contact with the insulator layer 830. Semiconductormaterial layer 850 is a conformal semiconductor material layer. Thesource and drain have conventionally accepted meanings, and either thefirst electrode 880 or the second electrode 870 can be designated thesource (or drain) as is required by the application or circuit. Thefirst electrode 880, second electrode 870 and third electrode 875 can bea single conductive material, as shown, or may comprise any number ofconductive material layers. The first electrode 880 is located incontact with a first portion of the semiconductor layer 850 over the topof the vertical-support-element 120. The second electrode 870 is locatedin contact with a second portion of the semiconductor layer 850 over thesubstrate 110 and not over the vertical-support-element 120, as shown inFIG. 8a . The first electrode 880 and second electrode 870 electrodedefine a first channel in the semiconductor layer between the firstelectrode 880 and second electrode 870. The elements of bottom-gate VTFT801 are the same as the elements of the top-gate VTFTs discussed early,serve the same function with the arrangement being that of a bottom-gaterather than a top-gate VTFT. As shown, transistors 801 and 802 have acommon gate and are formed in series, and their function should beunderstood from the description of transistors 100 and 200 of FIGS. 1aand 1b . From the description in the incorporated U.S. application Ser.No. 14/198,628, filed Mar. 6, 2014, it should also be understood thatmultiple variations of the bottom-gate VTFT formed over avertical-support-structure are possible, including independentlyoperable bottom-gate VTFTs formed over a singlevertical-support-element. Additionally, although the electricallyconductive gate structure 820 shown in FIGS. 8a and 8b is illustrated asconstructed from a vertical-support-element 120 and a separate conformalgate layer 825, in alternative embodiments electrically conductive gatestructure 820 can be formed by a vertical-support-element 120 formed ofconductive materials without the need for a separate gate layer 825.

Turning now to embodiments of the present invention having a bottom-gateand a top-gate vertical transistor formed over a commonvertical-support-structure, transistors 803 and 303 are illustrated bythe schematic cross-sectional view of FIG. 9a , taken along the lineA-A′ of the plan view shown in FIG. 9b . As shown, transistor 803 is abottom-gate vertical transistor and transistor 303 is a top-gatevertical transistor, where both transistors are formed over a singlevertical-support-element 120. The vertical-support-element 120 as shownin FIGS. 9a and 9b is an insulating structure, and the conductive layersare conformal and formed separately from the vertical-support-element120. As shown, there is an electronic device that includes a substratewith a vertical-support-element 120 on the substrate. Thevertical-support-element 120 extends away from the substrate 110 andincludes a first edge having a first reentrant profile 140 and a secondedge having a second reentrant profile 145. A top-gate verticaltransistor 303 is formed using first reentrant profile 140. There is afirst conformal semiconductor layer 150 in contact with thevertical-support-element 120 in the first reentrant profile 140. Thesource/drain electrodes of the top-gate VTFT 303 include a firstelectrode 180 located in contact with a first portion of the firstconformal semiconductor layer 150 over the top of thevertical-support-element 120 and a second electrode 170 located incontact with a second portion of the first conformal semiconductor layer150 over the substrate 110 and not over the vertical-support-element120, and adjacent to the first edge of the vertical-support-element 120.The gate dielectric of the top-gate VTFT 303 is a first conformaldielectric layer 130 on the first semiconductor layer 150 in the firstreentrant profile 140. There is a conformal conductive top-gate 125 onthe first conformal dielectric layer 130 in the first reentrant profile140.

Bottom-gate vertical transistor 803 is formed in the second reentrantprofile 145 of vertical-support-element 120. There is a conformalconductive bottom-gate 825 in the second reentrant profile 145 and incontact with the vertical-support-element 120. A second conformaldielectric layer 830 is in the second reentrant profile 145 and incontact with the conformal conductive bottom gate 825. Over the secondconformal dielectric layer 830, there is a second conformalsemiconductor layer 850 in the second reentrant profile 145 and incontact with the second conformal dielectric layer 830. The source/drainelectrodes of the bottom-gate VTFT 803 include a third electrode 875located over the substrate 110 and not over the vertical-support-element120, and adjacent to the second edge, the third electrode 875 being incontact with the second semiconductor layer 850, and a fourth electrode885 located over the top of the vertical-support-element 120 and incontact with the second semiconductor layer 850. The first electrode 180and the second electrode 170 define a first semiconductor channel of atop-gate vertical transistor 303, and the third electrode 875 and thefourth electrode 885 define a second semiconductor channel of abottom-gate vertical transistor 803.

As illustrated in FIGS. 9a and 9b , transistors 303 and 803 are formedover a common vertical-support-element 120 and can be formed from commonmaterial layers. The elements of each transistor 303 and 803 are shadedas to illustrate which elements are part of a common material layer. Asshown the first conformal top-gate layer 125 and the source/drainelectrodes of the bottom-gate VTFT 803, third electrode 875 and fourthelectrode 885, can be formed in a common material layer, although eachis electrically distinct. Similarly, as shown the second conformalbottom-gate layer 825 and the source/drain electrodes of the top-gateVTFT 303, first electrode 180 and second electrode 175, can be formed ina common material layer. As shown the first conformal semiconductorlayer 150 is a different material layer than the second conformalsemiconductor layer 850. In some embodiments, the first semiconductorlayer 150 and the second semiconductor layer 850 can be the samematerial. In preferred embodiments, transistors 303 and 803 are n-typetransistors, more preferably, n-type metal oxide thin film transistors.In preferred embodiments, both the first semiconductor layer 150 and thesecond semiconductor layer 850 include a ZnO-based semiconductor. Asshown, the first conformal dielectric layer 130 and the second conformaldielectric layer 830 are two separate portions of a common dielectriclayer, and as such have the same material composition and thickness.Preferably, the dielectric material is an insulating metal oxide.

Selective area deposition can be advantageously used to pattern portionsof layers in devices in which a conformal gate layer of one device andthe source and drain electrodes of a second device are formed over thesame vertical-support-element from a common material layer. In theseembodiments, the conformal conductive bottom-gate, the third electrode,and the fourth electrode have the same material composition andthickness and are part of the same conductive material layer.Preferably, the conductive material composition includes a conductivemetal oxide. The conductive material layer must be put down with aconformal deposition process, like ALD, in order to form the conformalgate. In order to pattern a conformal conductive material layer intoseparated source and drain electrodes on either side of a reentrantprofile, a deposition inhibitor can be used to fill in the reentrantprofile such that the conductive material is not deposited in theprofile. There is no easy photolithographic lift-off or etching processthat could yield the simultaneous creation of a conformal gate andseparated source/drain electrodes over the same VTFT structure.

FIG. 10c is a schematic cross-sectional view of FIG. 10b , taken alongthe line A-A′ of the plan view shown in FIG. 10b . As illustrated,bottom-gate VTFT 804 and top-gate VTFT 304 have the same elements asbottom-gate VTFT 803 and top-gate VTFT 303 discussed above with respectto FIGS. 9a and 9b . FIGS. 10a through 10c illustrate the embodimentwhere a bottom-gate VTFT 804 and top gate VTFT 304 are formed over thesame vertical-support-element 120 and are connected to operate as anenhancement-depletion-mode inverter. The equivalent circuit for anenhancement-depletion-mode inverter is shown in FIG. 10a , where T1 isthe load transistor and T2 is the drive transistor. In the configurationshown in illustrated in FIGS. 10a through 10c , the top-gate verticaltransistor 304 is the load transistor, T1, and the bottom-gate verticaltransistor 804 is the drive transistor, T2. The top-gate verticaltransistor 304 illustrated in FIGS. 10a through 10c operates in adepletion-mode, while the bottom-gate VTFT 804 operates in anenhancement mode. As shown, bottom-gate VTFT 804 and top gate VTFT 304share a common dielectric layer 130, with a portion 830 in the region ofthe channel of the bottom-gate VTFT 804. There is also a via 135 indielectric layer 130 to allow for electrical connection between thefirst electrode 180 and the conformal top-gate 125. The conformaltop-gate 125 is also extended over the top of thevertical-support-element 120 so as to connect to the fourth electrode885. As shown in FIG. 10c , the fourth electrode 885 and the conformaltop-gate 125 are formed from a single material pattern and layer, havingthe same material composition and layer thickness. As shown, there isalso an optional via in dielectric layer 130 to allow to surface contactto the second electrode 170; the use of this and other vias will bedictated by the contact requirements of the circuitry. The remainingelements of the VTFTs 304, 804 of FIGS. 10a through 10c are equivalentto those described with respect to FIGS. 9a and 9b , and should beunderstood from the previous description. The specific layout of theinverter is shown in FIGS. 10b and 10c is chosen for simplicity ofillustration. Many designs are possible and are within the scope of thecurrent invention as long as they meet the requirements of havingbottom-gate and a top-gate vertical transistors formed over a singlevertical-support-element 120, and are properly connected as in theequivalent circuit diagram of FIG. 10 a.

FIG. 11 is schematic cross-sectional view of an alternative embodimentwhere a bottom-gate VTFT 805 and top gate VTFT 305 are formed over thesame vertical-support-element 120 and are connected to operate as anenhancement-depletion-mode inverter. As shown bottom-gate VTFT 805 andtop gate VTFT 305 are equivalent to bottom-gate VTFT 804 and top gateVTFT 304 discussed with respect to FIG. 10a through 10c , and have thesame elements with the addition of another dielectric layer element 132.As shown, the first dielectric layer 130 and the second dielectric layer830 are part of a common shared dielectric layer (as in FIGS. 10athrough 10c ). As shown in FIG. 11, dielectric layer 130 and additionaldielectric layer 132 form dielectric stack 131. The dielectric stack 131can include a plurality of layers. Additional dielectric layer 132 ispatterned so that it adds additional gate-dielectric to top-gatetransistor 305, but not to bottom-gate transistor 805 such that thegate-dielectric of top-gate transistor 305 is thicker than thegate-dielectric 830 of bottom-gate VTFT 805. The dielectric stack 131should be considered as a common variable thickness dielectric stackshared between the top-gate and bottom-gate VTFT. As shown in FIG. 11,the full plurality of dielectric layers included in dielectric stack 131are present at some regions of the VTFTs (for example, dielectric layers130 and 132 are both present in reentrant profile 140), while only aportion of the plurality of layers included in dielectric stack 131 ispresent in other regions of the VTFTs (for example, only dielectriclayer 130 is present in reentrant profile 145. Although the variablethickness dielectric stack is illustrated here in anenhancement-depletion mode inverter arrangement, it should be understoodthat any arrangement of a bottom-gate VTFT and top-gate VTFT formed overthe same vertical-support-element can include this feature.

The vertical-support-element 120 of the present invention can be used toform dual-gate vertical transistors, which is to say transistors havinga gate on both sides of the semiconductor layer. Dual-gate devices areuseful to tune the performance of the transistor and generally havehigher on-currents than single gate devices. Shown in FIG. 12a is aschematic cross-sectional view of two dual-gate vertical transistors701,702, taken along the line A-A′ in the plan view shown in FIG. 12b .As shown, the electronic device includes a substrate 110, and anelectrically conductive gate structure 760 on the substrate 110. Theelectrically conductive gate structure 760 includesvertical-support-element 120 and a conformal conductive bottom-gatelayer 720. Preferably, the vertical-support-element 120 is formed from apost and a cap, where the cap extends beyond the edge of the post toform the first reentrant profile 140. As shown in FIG. 12a , theelectrically conductive gate structure 760 extends away from thesubstrate to a top, and the top extends beyond the first edge to definea first reentrant profile. A first conformal dielectric layer 715 is incontact with the electrically conductive gate structure 760 and part ofthe substrate 110. The first conformal dielectric layer 715 can be saidto maintain the reentrant profile 140. Next, there is a conformalsemiconductor layer 750 that also maintains the first reentrant profile140 and is in contact with the conformal electrically dielectric layer715. A first electrode 780 is located in contact with a first portion ofthe semiconductor layer 750 over the top of the electrically conductivegate structure 760, and a second electrode 770 is located in contactwith a second portion of the semiconductor layer 750, over the substrate110 and not over the top of the electrically conductive gate structure760, and adjacent to the first edge of the electrically conductive gatestructure 760. A second conformal dielectric layer 730 is on thesemiconductor layer 750 in the first reentrant profile 140. A conformalconductive top-gate 725 is on the conformal dielectric layer 730 in thefirst reentrant profile 140. The first electrode 780 and the secondelectrode 770 define a semiconductor channel of a dual-gate transistor701.

As shown in FIG. 12a , the dual-gate vertical TFT 701 is formed inseries with vertical TFT 702, and share both the top-gate 725 and thebottom-gate 720. In this configuration there are three electrodesincluding the shared first electrode 780 over thevertical-support-element 120, a second electrode 770 for TFT 701 notover the vertical-support-element 120, and a third electrode 775 for TFT702 not over the vertical-support-element 120. As shown, theelectrically conductive gate structure 760 has a second reentrantprofile 145. The third electrode 775 is located in contact with a thirdportion of the semiconductor layer 750 over the substrate 110 and notover the vertical-support-element 120, and adjacent to the secondreentrant profile 145.

The first electrode 780 and the second electrode 770 define the channelof the first TFT 701, and the third electrode 775 and the firstelectrode 780 define the channel of the second TFT 702. Theconfiguration shown in FIGS. 12a and 12b allows contact to the verticaltransistors 701 and 702 to be made on the substrate level so thetransistors are operated in series, rather than making connection at thetop of the electrically conductive gate structure 760. It should beunderstood that the present invention includes a single dual-gatevertical transistor where the electrodes are disposed as 770 and 780 inFIG. 12a . As shown, the electrically conductive gate structure 760functions as the bottom-gate and the electrically conductive top-gate725 functions as the top-gate for dual-gate transistors 701 and 702. Insome example embodiments of transistor 701, electrode 770 functions asthe drain of transistor 701 and electrode 780 functions as the source oftransistor 701. In other example embodiments of transistor 701,electrode 770 functions as the source and electrode 780 functions as thedrain. The semiconductor device is actuated in the following manner.After transistor 701 is provided, a voltage is applied between theelectrode 770 and the electrode 780. A voltage is also applied either tothe electrically conductive gate structure 720, or to the electricallyconductive top-gate 725, or to both to electrically connect theelectrode 770 and the electrode 780. In some embodiments theelectrically conductive gate structure 720 and the electricallyconductive top-gate 725, are electrically connected and operatedsimultaneously. In other embodiments, the top-gate acts as a biaselectrode, and transistors are operated with independent voltagesapplied to the electrically conductive gate structure 720 and theelectrically conductive top-gate 725.

The dual-gate vertical transistors can be configured such that the twodual-gate devices formed over a single vertical-support-element areindependently operable. FIG. 13 is a schematic cross-sectionalillustration of two independently operable dual-gate VTFTs 703, 704.They are similar in construction to the dual-gate VTFTs 701 and 702 inFIGS. 12a and 12b . As shown, there is an additional fourth electrode785. The fourth electrode 785 located in contact with a fourth portionof the semiconductor layer 750 over the top of the electricallyconductive gate structure 760, and electrically separate from the firstelectrode 780. The semiconductor layer has been patterned into tworegions including a first semiconductor region 750 of the firstdual-gate VTFT 703, and a second semiconductor region 751 of the seconddual-gate VTFT 704. The conformal conductive top-gate has been patternedto have two independent top-gates, a first conformal conductive top-gate725 and a second conformal conductive top-gate 727. Similarly, theconformal conductive bottom gate has two independently operable sectionsa first conformal conductive bottom-gate 720 and a second conformalconductive bottom-gate 721. In this embodiment, the first conformalconductive bottom-gate 720, the second conformal conductive bottom-gate721, and the vertical-support-element 120 form the electricallyconductive gate structure 760.

FIGS. 14a through 14c illustrate the embodiment where the two dual-gateVTFTs formed over a single vertical-support-element are arranged tooperate as an inverter. As shown, dual-gate VTFTs 705 and 706 are formedover the same vertical-support-element 120 and are connected to operateas an enhancement-depletion-mode inverter, having an additional node“Vtop” that can be used to drive transistor T2. The equivalent circuitfor a dual-gate enhancement-depletion-mode inverter is shown in FIG. 14b, where T1 is the load transistor and T2 is the drive transistor. Asshown, the top-gate and bottom-gate of the load transistor T1 areelectrically connected, while the top-gate and bottom of the drivetransistor T2 are electrically independent. In the configuration shownin illustrated in FIGS. 14a through 14c , the dual-gate verticaltransistor 705 is the load transistor, T1, and the dual-gate verticaltransistor 706 is the drive transistor, T2. The dual-gate verticaltransistor 705 illustrated in FIGS. 14a through 14c operates in adepletion-mode, while the dual-gate VTFT 706 is configured such that thetop-gate is a bias electrode to tune the enhancement mode operation ofthe drive transistor.

As shown, dual-gate VTFTs 705 and 706 share a two common dielectriclayers 715 and 730. There is a via 735 in both dielectric layers 715,730to allow for electrical connection between the first electrode 780 andthe conformal top-gate 725. The first electrode 780 is connected to thefourth electrode 785 over the top of the vertical-support-structure; andas shown in FIG. 14a , the fourth electrode 785 and the first electrode780 are formed from a single material pattern and layer, having the samematerial composition and layer thickness. The via 735 in dielectriclayer 715 allows the first electrode 780 to be connected to theconformal bottom-gate of the load transistor, dual-gate VTFT 705. Asshown, there are additional optional vias in dielectric layer 730 toallow surface contact to the second and third electrode 770 and 785; theuse of this and other vias will be dictated by the contact requirementsof the circuitry. The remaining elements of the dual-gate VTFTs 705 and706 are equivalent to those described with respect to VTFTs 701,702 and703,704 of FIGS. 12a , 12, and 13, and should be understood from theprevious description. The specific layout of the dual-gate inverter isshown in FIGS. 14a and 14c is chosen for simplicity of illustration.Many designs are possible and are within the scope of the currentinvention as long as they meet the requirements of having two dual-gatetransistors formed over a single vertical-support-element 120, and areproperly connected as in the equivalent circuit diagram of FIG. 14 b.

FIG. 15a illustrates an equivalent circuit for an all-enhancement-modeinverter. An all enhancement-mode inverter includes the first and secondtransistors T1 and T2. The first transistor T1 functions as a loadtransistor, and the second transistor T2 functions as a switching ordrive transistor. Both the first TFT T1 and the second TFT T2 areoperated in an enhancement mode.

As described above, an enhancement mode device is one that is normallyoff, and has a threshold voltage greater than zero. Each of the firstand second transistors T1 and T2 includes a gate, a gate dielectriclayer, a semiconductor layer, and source and drain electrodes. Thesemiconductor layer can be made of a semiconducting metal oxidematerial, and is preferably a ZnO-based semiconductor, for example, butnot limited to, ZnO or InGaZnO₄. As shown in an all-enhancement-modeinverter, the source of the load transistor T1 is electrically connectedto the drain of the drive transistor T2, and the drain and gate of theload transistor T1 are electrically connected.

FIG. 15b illustrate a cross-sectional view of two bottom-gate VTFTs on acommon vertical-support-element configured to operate as anall-enhancement-mode inverter. The specific layout of the inverter isshown in FIG. 15b was chosen for simplicity of illustration. Manydesigns are possible and are within the scope of the current inventionas long as they meet the requirements of having two transistors formedover a single vertical-support-element 120, and are properly connectedas in the equivalent circuit diagram of FIG. 15 a.

As shown in FIGS. 15b , an enhancement-mode inverter of the presentinvention has a load transistor T1 that is bottom-gate VTFT 305 with afirst source 380, a first drain 370, a load channel region in thesemiconductor layer 350, a load gate dielectric in the load channelregion of the dielectric layer 330 of VTFT 305 and a first gateelectrode 325. The drive transistor 315 of the enhancement-mode inverterof the present invention has a bottom-gate VTFT architecture with asecond source 375, a second drain 385, a drive channel region in thesemiconductor layer 350, a drive gate dielectric in the drive channelregion of the dielectric layer 330 of VTFT 315 and a second gateelectrode 327. The load and drive transistors 305, 315 are connected asin the equivalent circuit shown in FIG. 15a ; the first source 380 iselectrically connected to the second drain 385 and the first gate 325 iselectrically connected to the first drain 370. The load and drive TFTs305, 315 have a common shared dielectric layer 330, where the shareddielectric layer can be a single layer or a multilayer dielectric stack.As shown, TFTs 305 and 315 are on a common vertical-support-element 120,and have their individual elements formed in common layers.

FIG. 16 illustrates an enhancement-mode inverter formed on a singlevertical-support-element where tuning the relative current-carryingability of the drive and load TFTs can be accomplished using the gatedielectric thickness of each TFT in addition to the W/L dimensions oftheir channels. The use of a common variable thickness dielectric stackas shown in FIG. 16 has the advantage of providing another dimension foroptimizing the inverter and circuit design. Unlike standard circuitdesigns, which are limited to using the channel dimensions of the TFTs,the use of the variable thickness common dielectric stack allows theperformance of the drive and load TFTs 316, 306 to be independentlytuned using the thickness of the drive gate dielectric and the load gatedielectric. The common dielectric stack has an additional layer 335 inthe region of the channel of bottom-gate VTFT 306.

Some embodiments of the present invention relate to planar top-gate thinfilm transistors (TFTs) which, as previously described, have asemiconductor layer that is between the substrate and the gate layer.Unlike bottom-gate TFTs, top gate TFTs are inherently protected from theenvironment since the semiconductor layer is between the substrate andthe gate dielectric. When forming top-gate planar TFTs on somesubstrates, including inorganic substrates, the back-channel interfaceproperties can cause the device to operate in the depletion mode.

FIG. 17a is a cross-sectional diagram of a prior art planar top-gate TFT900, taken along the line A-A′ of the plan view shown in FIG. 17c . TheTFTs 900 shown in FIGS. 17a and 17b is a planar top-gate device that isrepresentative of any planar top-gate TFT 900, having source and drainelectrodes 980,985 in contact with a substrate 910, a semiconductorlayer 970 that is over and in contact with the source/drain 980/985, adielectric layer 930 over and in contact with semiconductor layer 970,and a gate 920 over and in contact with the dielectric layer 930, aso-called staggered structure. FIG. 17b illustrates a differentembodiment of a typical top gate TFT where the semiconductor layer 970is under the source and drain electrodes, in a coplanar structure. Theremainder of the device is constructed as shown in FIG. 17a . Thesubstrate 910 can be any previously discussed substrate, and can containa plurality of predefined layers. The source and drain haveconventionally accepted meanings, and either electrode shown can bedesignated the source (or drain) as is required by the application orcircuit. The source and drain electrodes 980,985 can be a singleconductive material, as shown in FIGS. 17a and 17b or can comprise anynumber of conductive material layers. The gap between the source anddrain electrodes 980, 985 defines the semiconductor channel of planartop-gate TFT 900, as is conventionally understood. The inorganicsemiconductor layer 970 is in contact with both the source and drainelectrodes 980, 985 and the substrate 910, as shown in FIG. 17a . Theinorganic semiconductor layer 950 can be a metal oxide, for example aZnO-based material. The dielectric layer 930 can be a single layer ofdielectric, as shown, or in embodiments of the present invention thedielectric layer 930 is formed from a variable thickness dielectriclayer. The depletion-mode performance of the planar top-gate TFT isuseful in combination with the enhancement mode planar or verticalbottom-gate TFTs to construct enhancement-depletion-mode inverters andcircuits.

FIG. 18a is a cross-sectional diagram of a prior art bottom gate TFT 1,taken along the line A-A′ of the plan view shown in FIG. 18b . The TFT 1shown in FIGS. 18a and 18b is a planar bottom-gate device that isrepresentative of any planar bottom-gate TFT 1, having a gate 12 incontact with a substrate 110, a dielectric layer 13 over and in contactwith the gate 12, and a semiconductor layer 15 over and in contact withthe dielectric layer 13. The source and drain electrodes 18, 19 are overand in contact with the semiconductor layer 15 in a coplanar geometry.FIGS. 19a and 19b illustrate a different embodiment of a typical planarbottom-gate TFT 2 where the semiconductor layer 15 is under the sourceand drain electrodes 18, 19 in a staggered geometry. The remainder ofthe device is constructed as shown in FIGS. 18a and 18b . The substrate110 can be any previously discussed substrate, and can contain aplurality of predefined layers. The source and drain have conventionallyaccepted meanings, and either electrode shown can be designated thesource (or drain) as is required by the application or circuit. Thesource and drain electrodes 18, 19 can be a single conductive material,as shown in FIGS. 18a and 19b or can comprise any number of conductivematerial layers. The gap between the source and drain electrodes 18, 19defines the channel of TFT 1 or 2, as is conventionally understood. Theinorganic semiconductor layer 15 is in contact with both the source anddrain electrodes 18, 19 and the dielectric layer 13. Bottom-gate TFT 1or 2 can be passivated, meaning TFT 1 or 2 can have an additional layeron the side of the semiconductor layer 15 that is not in contact withthe dielectric, the so-called back-side of the semiconductor layer 15,to protect the device for environmental effects. The inorganicsemiconductor layer 15 can be a metal oxide, for example a ZnO-basedmaterial. The dielectric layer 13 can be a single layer of dielectric,as shown, or in embodiments of the present invention the dielectriclayer 13 is formed from a variable thickness dielectric layer.Bottom-gate ZnO-based TFTs can operate in enhancement-mode, meaning thatthey are normally off. The enhancement-mode performance of planarbottom-gate TFTs is useful in combination with the otherenhancement-mode bottom-gate planar TFTs and VTFTs to constructall-enhancement-mode inverters and circuits, and in combination withtop-gate planar TFTs and VTFTs to construct enhancement-depletion-modeinverters and circuits.

FIGS. 20a through 20d are schematic cross-sectional views of planarbottom-gate TFTs illustrating different useful configurations amultilayer dielectric stack for use as the dielectric layer. Althoughillustrated in a planar bottom-gate format, the various embodiments ofthe multilayer dielectric stack are useful in other TFT architectures tovary the thickness of the gate dielectric and thickness of thecross-over dielectric to tune individual device performance. Forexample, previously discussed FIG. 11 illustrates the use of a variablethickness dielectric stack in a device having a bottom-gate VTFT and atop-gate VTFT formed over the same vertical-support-element 120.Returning to FIG. 20a through 20d , each Figure illustrates abottom-gate planar TFT having the same elements as the planarbottom-gate TFT 2 described in relationship to FIGS. 19a and 19b wherethe single layer dielectric shown as 14 has been replaced with amultilayer dielectric stack 23. The gate 12, source/drain 18/19,semiconductor 15, and substrate 110 should be understood from previousdescriptions.

Although multilayer dielectric stack 23 is shown in FIG. 20a as having 2layers, multilayer dielectric stacks can have any number of layers asdesired. Preferably the dielectric layers 21 and 22 are insulating thinfilm inorganic material layers, more preferably layers 21 and 22 areformed from the same material. As shown in FIG. 20a , the multilayerdielectric stack 23 can be formed from multiple layers each having thesame pattern. FIGS. 20b through 20c illustrate alternative architectureswhere the multilayer dielectric stack is composed of multiple layershaving different patterns.

FIG. 20b illustrates a multilayer dielectric stack 23 with two layers 22and 25, however TFT 4 has a gate dielectric with a thickness that is thesame as the thickness of the second dielectric layer 22. As shown inFIG. 20b , dielectric layer 25 has been patterned to have a via 14 inthe channel region of TFT 4, such that the second dielectric layer 22defines the gate dielectric thickness, and dielectric protection layer25 protects against shorting in the overlap region between thesource/drain 18/19 and the gate 12 outside of the channel region of TFT4.

FIG. 20c has a three layer 25, 22, 26 multilayer dielectric stack 23where the second layer 22 and dielectric protection layer 25 areequivalent to those in FIG. 20b . TFT 5 of FIG. 20c has a buffer layer26 as part of the multilayer dielectric stack 23. Buffer layer 26 is adielectric layer having the same x-y pattern as the semiconductor 15.Buffer layer 26 is used in devices formed by selective area depositionprocesses to control the interface between the gate dielectric(multilayer dielectric stack 23) and the semiconductor 14. FIG. 20dillustrates an alternative embodiment of a buffer layer 26 incombination with the dielectric protection layer 25, without otherdielectric layers in the multilayer dielectric stack in the region ofTFT 6. As shown in FIG. 20d the gate dielectric thickness of TFT 6 isdefined by the thickness of the buffer layer 26. Although buffer layer26 and the multilayer dielectric stack 23 are illustrated in FIGS. 20athrough 20d in planar bottom-gate TFTs, they are also useful forvertical bottom-gate transistors.

All of the thin-film transistors described thus far are preferablycomposed of thin-film inorganic material layers. Each transistorarchitecture can be fabricated using the combination of SALD andselective area deposition (SAD). Using selective area deposition incombination with ALD to pattern thin-film inorganic layers has thebenefit of being an additive patterning technology, where there is noneed to be concerned with the relative etch rates of the differentmaterials. Preferably each of the layers of the transistors illustratedare formed from metal oxides, as described earlier. When using anadditive patterning technique, like SALD in combination with SAD,multiple types of architectures can easily be fabricated on the samesubstrate. Transistors of different architectures can be chosen fortheir individual performance attributes, for example as discussed inpreviously incorporated by reference U.S. application Ser. No.14/526,634, filed Oct. 29, 2014, planar top-gate and bottom-gatetransistors can be integrated to form enhancement-depletion modecircuitry. As illustrated in FIGS. 10a through 10c and FIG. 11, top-gateand bottom-gate VTFTs can be built over a singlevertical-support-element and can share common patterned material layers.In circuit design, it is usually necessary to vary the width (W) andlength (L) of the semiconductor channel of different elements in thecircuitry on a single substrate. Vertical transistors typically haveshort channel lengths, which are advantaged for high current needs.Planar transistors typically have longer channels than verticaltransistors, when high-end photolithography is not employed inprocessing. In some applications, the use of long channel verticaltransistors is useful to adjust W and L to achieve the desired relativeTFT performance. The ability to mix and match vertical and planartransistors, with both top-gate and bottom-gate architectures, is usefulfor the ease of sizing transistor components in complex circuitry. Bothlong channel vertical transistors and planar transistors have channellengths that are primarily (or fully in the case of a planar TFT)defined by the length of the channel that is parallel to the substrate.Electronic components with mixed devices have a first transistor whereat least a portion of the first semiconductor channel extends adirection parallel to the substrate, a vertical-support-element, and asecond transistor with a channel having at least a portion extending ina direction orthogonal to the substrate. The length of the firstsemiconductor channel is shorter than the length of the firstsemiconductor channel.

Table 1 illustrates different transistor architectures formed fromcommon material layers over a single substrate. In Table 1, staggeredcontacts are abbreviated as stag, coplanar contacts are abbreviated ascp, bottom-gate is abbreviated BG, top-gate is abbreviated TG, andvertical-support-element is abbreviated as V-S-E.

TABLE 1 BG- BG- TG- TG- DUAL- BG- BG- TG- TG- ID Layer VTFT VTFT VTFTVTFT VTFT planar planar planar planar Contact stag cp stag cp stag stagcp stag cp geometry: A Thick V-S- V-S- V-S- V-S- V-S-E — — — — InsulatorE E E E B Conductive Gate Gate S/D — Gate Gate Gate S/D — Layer CSemiconductor — — Semi Semi — — — Semi Semi Layer D Dielectric Diel DielDiel — Diel Diel Diel Diel — Layer E Semiconductor Semi — — — Semi Semi— — — Layer F Conductive S/D S/D Gate S/D S/D S/D S/D Gate S/D Layer GDielectric — — — Diel Diel — — — Diel Layer H Conductive — — — Gate TG —— — Gate Layer I Semiconductor — Semi — — — — Semi — — Layer

Table 1 illustrates 9 different transistor architectures that arepossible to make on the same substrate from the combination of 9different layers. With the exception of layer A—the thick insulator usedto form the vertical-structure-element, the all remaining layers (B-I)are preferably conformal thin-film inorganic layers that were depositedand patterned using the combination of SALD and SAD. Although denoted inTable 1 as layers, each layer in Table 1 could be a single layer or amultilayer stack of the same or different materials as long as the finalstack has the properties associated with the layer in Table 1. Table 1is not limiting in the number of common or shared layers possible inconstructing transistors of various architectures on a single substrate.That is, additional layers can be added prior to layer A, after layer I,or in between any of the layers listed in Table 1. For example, it maybe desirable to have a patterned conductive layer on the substrate priorto forming layer A to act as the primary power lines for a givencircuit. In some circumstances, it is desirable to form a passivationlayer after forming the last semiconductor layer I. Furthermore, it isnot required that all of the layers A through I are present on thesubstrate, only that there is a sufficient number of layers to form thedesired elements. In one embodiment, there is a bottom-gate VTFT and abottom-gate planar TFT formed on the same substrate, both having astaggered arrangement for the source/drain electrode. In thisembodiment, only 5 common layers A, B, D, E and F are required to formthe elements of the two transistor architectures.

Returning to the Figures, FIGS. 21 through 24 are schematiccross-sectional illustrations of various embodiments where a verticaltransistor and a planar transistor are formed on a common substrate. Asshown, there is an electronic component comprising two transistors on acommon substrate. The first transistor on the substrate being a planartransistor having a first semiconductor channel having a first lengthextending in a direction parallel to the substrate, the first transistorhaving a first source, a first drain, a first gate dielectric, and afirst gate. A vertical-support-element having a first reentrant profileis on the substrate, the vertical-support-element being a structuralelement used to form the vertical transistor. The second transistor onthe substrate is a vertical transistor having a second semiconductorchannel, including at least a portion of the second semiconductorchannel extending in a direction orthogonal to the substrate in thefirst reentrant profile of the vertical-support-element. The secondsemiconductor channel of the vertical transistor has a second lengththat is less than the first length. The second transistor has a secondsource, a second drain, a second gate dielectric and a second gate. Asused here the length of a semiconductor channel is the commonly accepteddefinition, meaning the shortest distance between the source and thedrain electrodes as drawn along the semiconductor surface.

Turning first to FIG. 21, the first transistor 410 is a planarbottom-gate TFT and the second transistor 420 is a bottom-gate VTFT. Theelements of first transistor 410 are the same as the elements of TFT 1illustrated in FIG. 18a , and are labeled the same for clarity. As shownthe dielectric layer 13 over the gate 12 of the first transistor 410, isthe first gate-dielectric of first transistor. The second transistor 420has the same elements of bottom-gate VTFT 801 illustrated in FIG. 8a ,and is similarly labeled with the same part numbers. Thevertical-support-element 120 has a first reentrant profile 140 and asecond reentrant profile 145. The first electrode 880 and the secondelectrode 870 are the second source and second drain, respectively, ofthe second transistor 420. The dielectric layer 830 in the region of thechannel of the second transistor is the second gate dielectric. Thesecond semiconductor channel should be understood from the previousdescriptions, and is defined by the second source and second drainelectrodes.

As illustrated in FIG. 21, there is a third transistor 430 whose channelis in the second reentrant profile 145. Third transistor 430 has a thirdsemiconductor channel, where at least a portion of the thirdsemiconductor channel extends in a direction orthogonal to the substratein the second reentrant profile. The third transistor 430 has a thirdsource, a third drain, a third gate dielectric, and a third gate. Thethird source is third electrode 875, and the third drain is the portionof the first electrode 880 adjacent the second reentrant profile 145over the vertical-support-structure 120. The second drain and the thirdsource are electrically connected and physically connected as they aretwo portions of the first electrode 880. The second gate and the thirdgate are two portions of the conductive conformal gate layer 825 in thefirst and second reentrant profiles 140 and 145 respectively.

As shown in FIG. 21, the second vertical transistor and the first planartransistor have 4 common layers, layers B, D, E and F from Table 1 andlayer A which has been patterned so that it is only present in theregion of the second vertical transistor. Common layers are indicated bycommon shading in the figure. As shown in FIG. 21, thevertical-support-element 120 is patterned in layer A. The first gate 12of planar bottom-gate TFT 410 and the second gate in conformalconductive gate layer 825 of bottom-gate TFT 420 are separate regions ofa common conformal conductive layer, layer B. The first dielectric 14 ofplanar bottom-gate TFT 410 and second dielectric portion of dielectric830 of vertical bottom-gate TFT 420 are separate regions of a commonconformal dielectric layer, layer D. Similarly, the first semiconductor15 planar bottom-gate TFT 420 and second semiconductor portion ofsemiconductor 850 of vertical bottom-gate TFT 420 are separate regionsof a common conformal semiconductor layer, layer E. Finally, the firstsource and first drain 18, 19 of planar bottom-gate TFT 410 and secondsource 880 and second drain 870 of vertical bottom-gate TFT 420 areseparate regions of a common conformal conductive layer, layer F.

FIG. 22 illustrates another embodiment of the present invention wherethe first transistor 411 is a planar top-gate TFT and the secondtransistor 421 is a bottom-gate VTFT. The vertical-support-element 120of FIG. 22 has a second reentrant profile 145 with a third verticaltransistor 431. The second transistor 421 and third transistor 431 areequivalent to the second and third transistors 420,430 of FIG. 21 andshould be understood from the previous description. The first transistor411 has the same elements as the top-gate planar transistor 900illustrated in FIG. 17b . As shown the dielectric layer 930 under thegate 920 of the first transistor 411, is the first gate dielectric offirst transistor 411. The inorganic semiconductor layer 950 is incontact with both the source and drain electrodes 980, 985.

As shown in FIG. 22, the second vertical transistor and the first planartransistor are formed from 6 layers (layers A, AA, B, D, E and F) andhave 3 common layers, layers B, D, and F from Table 2. Alternatively,the transistors shown in FIG. 22 could be made from layers A, B, C, D,E, F, G and H from Table 1. Common layers are indicated by commonshading in the figure. As shown in FIG. 22, the vertical-support-element120 is formed in layer A. The first semiconductor 970 of the firsttransistor 411 is in layer AA. The first source 980 and first drain 985of planar top-gate TFT 411 and the second gate in conformal conductivegate layer 825 of bottom-gate TFT 421 are separate regions of a commonconformal conductive layer, layer B. The first dielectric 930 of planarbottom-gate TFT 411 and second dielectric 830 of vertical bottom-gateTFT 421 are separate regions of a common conformal dielectric layer,layer D. The second semiconductor portion of semiconductor 850 ofvertical bottom-gate TFT 421 is in layer E. Finally, the first gate 920of planar top-gate TFT 411 and second source 880 and second drain 870 ofvertical bottom-gate TFT 421 are separate regions of a common conformalconductive layer, layer F.

TABLE 2 ID Layer BG-VTFT TG-planar BG-VTFT TG-planar stag cp stagstaggered A Thick Insulator V-S-E — V-S-E — AA Semiconductor Layer —Semi — — B Conductive Layer Gate S/D Gate S/D C Semiconductor Layer — —— Semi D Dielectric Layer Diel Diel Diel Diel E Semiconductor Layer Semi— Semi — F Conductive Layer S/D Gate S/D Gate

In an alternative embodiment of a bottom-gate VTFT and a top-gate planarTFT formed on the same substrate, a top-gate planar TFT having astaggered structure can be used. In this embodiment the top-gate planartransistor would be replaced with a top-gate planar having thearchitecture illustrated in FIG. 17a . In this embodiment, the firstplanar top-gate TFT and second bottom-gate VTFT can be formed from 6layers, A, B, C, D, E and F from Table 1 or 2.

The embodiments illustrated in FIGS. 23 and 24 are the analogs of theembodiments shown in FIGS. 21 and 22 where the second transistor is atop-gate VTFT. As shown in FIG. 23, an electronic element has a firsttransistor that is a planar bottom-gate TFT and a second transistor thatis a top-gate VTFT. The elements of first transistor 410 are the same asthe elements of TFT 410 illustrated in FIG. 21 and should be understoodfrom the previous description. The second transistor 423 has the sameelements of top-gate VTFT 100 illustrated in FIG. 1a , and is similarlylabeled with the same part numbers. The vertical-support-element 120 hasa first reentrant profile 140 and a second reentrant profile 145. Thefirst electrode 180 and the second electrode 170 are the second sourceand second drain, respectively, of the second transistor 423. Thedielectric layer 130 in the region of the channel of the secondtransistor 423 is the second gate dielectric. The second semiconductorchannel should be understood from the previous descriptions, and isdefined by the second source and second drain electrodes 180, 170.

As illustrated in FIG. 23, there is a third transistor 433 whose channelis in the second reentrant profile 145. Third transistor 433 has a thirdsemiconductor channel, where at least a portion of the thirdsemiconductor channel extends in a direction orthogonal to the substratein the second reentrant profile. The third transistor has a thirdsource, a third drain, a third gate dielectric, and a third gate. Thethird source is third electrode 175, and the third drain is the portionof the first electrode 180 adjacent the second reentrant profile 145over the vertical-support-structure 120. The second drain and the thirdsource are electrically connected and physically connected, as they aretwo portions of the first electrode 180. The second gate and the thirdgate are two portions of the conductive conformal gate layer 125 in thefirst and second reentrant profiles 140, 145, respectively.

As shown in FIG. 23, the second top-gate vertical transistor 423 and thefirst bottom-gate planar transistor 410 can be formed from 6 layers, A,B, C, D, E and F from Table 1. Common layers are indicated by commonshading in the figure. As shown in FIG. 23, the vertical-support-element120 is formed in layer A. The first gate 12 of planar bottom-gate TFT410 and the second source 180 and second drain 170 of top-gate VTFT 423are separate regions of a common conformal conductive layer, layer B.The second semiconductor portion of semiconductor 150 of verticaltop-gate TFT 423 is in layer C. The first dielectric 13 of planarbottom-gate TFT 413 and second dielectric portion of dielectric 130 ofvertical bottom-gate TFT 423 are separate regions of a common conformaldielectric layer, layer D. The first semiconductor 15 of planarbottom-gate TFT 413 is in a semiconductor layer, layer E. Finally, thefirst source and first drain 18, 19 of planar bottom-gate TFT 413 andconformal conductive gate of vertical top-gate TFT 423 are separateregions of a common conformal conductive layer, layer F.

FIG. 24 is illustrative of the embodiment of the present invention wherea second top-gate VTFT 424 is on a common substrate with a firsttop-gate planar TFT 414. Top-gate planar TFT 404 has the same elementsas the top-gate planar TFT 401, and should be understood from theprevious description. The second transistor 424, is equivalent to thetop-gate VTFT 104 described with respect to FIG. 2c , and should beunderstood from the previous description. As illustrated, first planartop-gate TFT 404 and the second top-gate VTFT 424 share 5 common layers,A, C, F, G and H from Table 1. Common layers are indicated by commonshading in the figure. As shown in FIG. 24, the vertical-support-element120 is formed in layer A. The first semiconductor 950 planar top-gateTFT 414 and second semiconductor portion of semiconductor 150 ofvertical top-gate TFT 424 are separate regions of a common conformalsemiconductor layer, layer C. The first source and first drain 980,985of planar top-gate TFT 414 and second source 180 and second drain 170 ofvertical top-gate TFT 424 are separate regions of a common conformalconductive layer, layer F. The first dielectric 930 of planarbottom-gate TFT 414 and second dielectric portion of dielectric 130 ofvertical top-gate TFT 424 are separate regions of a common conformaldielectric layer, layer G. Finally, first gate 920 of planar top-gateTFT 414 and the second gate in conformal conductive gate layer 125 oftop-gate VTFT 424 are separate regions of a common conformal conductivelayer, layer H.

FIGS. 25a through 25c are illustrative of an embodiment of the presentinvention where the bottom-gate VTFTs 425, 435 and the planarbottom-gate TFT 415 are configured to operate as an all-enhancement-modeinverter. As illustrated, bottom-gate VTFT 425 is formed over the samevertical-support-element 120, and in series with, bottom-gate VTFT 435.This arrangement of using two VTFTs in series as an equivalent drivetransistor for an all-enhancement-mode inverter is shown in theequivalent circuit of FIG. 25a . Using two VTFTs in series over a singlevertical-support-element as an equivalent drive transistor is useful inany logic gate or circuit where the drive TFT is vertical TFT (a shortchannel device) and the load TFT is a planar device. The bottom-gateVTFTs 425 and 435 are equivalent to the bottom-gate VTFTs 420 and 430 ofFIG. 21, and should be understood from the previous description.Similarly, the bottom-gate planar TFT 415 is equivalent to thebottom-gate planar TFT 410 of FIG. 21, and should be understood from theprevious description. As with FIG. 21, the three transistors on thecommon substrate 110 of FIGS. 25b and 25c are formed from 5 layers, witheach with elements in 4 common layers B, D, E and F from Table 1. Asshown in FIGS. 25b and 25c , the first source 18 of the first planarbottom-gate TFT 415 is electrically and physically connected to thedrain/source 870 of the second TFT 425. The first drain 19 of the firstplanar bottom-gate TFT 415 is connect to its own gate 12, through a viain the common dielectric layer, as shown by an X in FIG. 25 b.

FIG. 26 is illustrative of an embodiment of the present invention wherethe bottom-gate VTFT 426 and the bottom-gate transistor 416 areconfigured to operate as an all-enhancement-mode inverter. Thebottom-gate transistor 416 is a long channel vertical transistor, wherethe length of the semiconductor channel is primarily determined by theportion of the channel that is parallel to the substrate. This is to saythat the length of the portion of the first semiconductor channelextending in the direction parallel to the substrate is greater than thelength of the portion of the first semiconductor channel extending inthe direction orthogonal to the substrate. Preferably the length of theportion of the first semiconductor channel extending in the directionparallel to the substrate is at least 10 times greater than the lengthof the portion of the first semiconductor channel extending in thedirection orthogonal to the substrate. The long-channel bottom-gate VTFT416 has a channel dimension that is typical of a planar transistor, buthas source and drain electrodes that are at different distances from thesubstrate (making it a vertical transistor). The benefit of usinglong-channel bottom-gate VTFT 416 over a planar transistor is areduction in the overall footprint of the device, the ability toleverage the second reentrant profile. The architecture shown in FIG. 26is advantaged over standard VTFTs in that length of the channel of VTFT416 can be controlled independent of the height of thevertical-support-element. The all-enhancement-mode inverter of FIG. 26has an equivalent circuit shown in FIG. 15a (standard equivalent circuitof an enhancement-mode inverter). As shown in FIG. 26, the firstbottom-gate TFT 416 has a first semiconductor channel with a portionextending in a direction orthogonal to the substrate in the secondreentrant profile 146 of vertical-support-element 120. The length of theportion of the semiconductor channel that is parallel to the substrateis much longer than the portion in the second reentrant profile 146,preferably the parallel portion is at least 10 times longer. It can besaid the length of the semiconductor channel of VTFT 416 ispredominately defined by the portion of the channel that is parallel tothe substrate. As shown in FIG. 26, the first reentrant profile 141 isused to form the second vertical bottom-gate transistor 426, having asecond source 871 and second drain 880. The first source 18 is on top ofthe vertical-support-element 120 adjacent to the second reentrantprofile 146, and electrically and physically connected to the seconddrain 880. The two transistors on the common substrate 110 of FIG. 26are formed from 5 common layers, A, B, D, E and F from Table 1.

FIG. 27 is illustrative of an embodiment of the present invention wherethe bottom-gate VTFT 427 and the long-channel top-gate VTFT 417 areconfigured to operate as an enhancement-depletion-mode inverter. Theenhancement-depletion-mode inverter has the standard equivalent circuitshown in FIG. 10a . As shown in FIG. 27, the first top-gate TFT 417 hasa first semiconductor channel with a portion extending in a directionorthogonal to the substrate in the second reentrant profile 146 ofvertical-support-element 120. The embodiment illustrated in FIG. 27 issimilar to the embodiment of an enhancement-depletion-mode inverterillustrated in FIGS. 10a through 10c of a bottom-gate VTFT 804 and atop-gate VTFT 304 that share a common vertical-support-element. In FIG.27, the top-gate depletion-mode TFT 417 is a long-channel top-gate VTFTsince its channel length is predominately determined by the portion ofthe semiconductor layer that is parallel to the substrate rather thanthe portion of the portion that is in the reentrant profile 146.Preferably, 90% or greater of the length of the semiconductor channel ofthe long-channel top-gate VTFT 417 is parallel to the substrate. Theelements of FIG. 27 are the same as the elements of FIG. 22. The secondvertical transistor 427 and the first transistor 417 have 6 layers, with4 common layers and are on a common substrate 110. The first source 980is electrically connected to the second drain 880 and the first gate920.

FIG. 28 is an alternative embodiment having bottom-gate vertical TFT 428on a common substrate with a long-channel top-gate VTFT 418. Theelements of the electronic device of FIG. 28 are the same as those inFIG. 27, with the addition of additional dielectric layer 435 inlong-channel top-gate VTFT 418 such that the electronic device has avariable thickness dielectric stack.

EXAMPLES

The preparation of a thin film coating of the material layers on glasssubstrates, as used in the examples, is described below. The ALD coatingdevice used to prepare these layers, namely aluminum oxide, ZnO:N, andAl-doped ZnO (AZO), has been described in detail in US PatentApplication Publication No. US 2009/0130858, the disclosure of which isherein incorporated by reference in its entirety. The coating device hasan output face (facing up) that contains spatially separated elongatedgas channels and operates on a gas bearing principle. The coating devicecan be understood with respect to delivery head 96 shown in FIG. 29.Each gas channel is composed of an output slot 95, 93, 92 which suppliesgas to the output face 94, and adjacent exhaust slots 91 which removegas from the output face 94. The order of the gas channels isP-O-P-M-P-O-P-M-P-O-P where P represents a purge channel, O represents achannel containing an oxygen based precursor, and M represents a channelcontaining a metal based precursor. As a substrate moves relative to thecoating head it sees the above sequence of gases which results in ALDdeposition.

A 2.5 by 2.5 inch square (63.5 by 63.5 mm square) glass substrateattached to a heated backer is positioned above the output face of thecoating device and is maintained in close proximity to the output faceby an equilibrium between the pull of gravity, the flow of the gasessupplied to the output face, and a slight amount of vacuum produced atthe exhaust slot. For all of the examples, the exhaust slot pressure wasapproximately 40 inches of water below atmospheric pressure. The purgegas P is composed of pure nitrogen. The oxygen reactive precursor O is amixture of nitrogen, water vapor, and optionally, ammonia vapor. Themetal reactive precursor M can be one active metal alkyl vapor, or amixture of active metal alkyl vapors, in nitrogen.

The metal alkyl precursors used in these examples were dimethylaluminumisopropoxide (DMAI) and diethyl zinc (DEZ). The flow rate of the activemetal alkyl vapor was controlled by bubbling nitrogen through the pureliquid precursor contained in an airtight bubbler by means of individualmass flow control meters. This saturated stream of metal alkyl was mixedwith a dilution flow before being supplied to the coating device. Theflow of water vapor was controlled by adjusting the bubbling rate ofnitrogen passed through pure water in a bubbler. This saturated streamof water vapor was mixed with a dilution flow before being supplied tothe coating device. The flow of ammonia vapor was controlled by passingpure ammonia vapor from a compressed fluid tank through a mass flowcontroller and mixing with the water vapor stream. All bubblers wereheld at room temperature. The temperature of the coating was establishedby controlling heating of both the coating device and the backer to adesired temperature. Experimentally, the flow rates of the individualgasses were adjusted to the settings shown in Table 3 for each of thematerial layers coated in the examples contained herein. The flows shownare the total flows supplied to the coating device, and thus arepartitioned equally among the individual gas channels.

The coating process was then initiated by oscillating the substrateacross the coating head for the number of cycles necessary to obtain auniform deposited film of the desired thickness for the given example.The coating head as described above contains two full ALD cycles (twooxygen and two metal exposures per single direction pass over the head),therefore a round trip oscillation represents 4 ALD cycles. All sampleswere coated at a substrate temperature of 200° C. and a residence timeof 50 ms.

TABLE 3 N₂ dilution N₂ DMAI TMA DEZ Water with dilution N₂ bubblerbubbler bubbler NH3 bubbler Metal with Inert flow flow flow flow flowAlkyl water Purge Layer (sccm) (sccm) (sccm) (sccm) (sccm) (sccm) (sccm)(sccm) Al₂O₁ 65  0  0 0 65   1500 2250 3000 Al₂O₁  0 30  0 0 22.5 15002250 3000 ZnO  0  0 60 0 45   1500 2250 3000 AZO 10  0 30 0 22.5 15002250 3000Vertical TFT Experiments

The top-gate VTFTs of the present invention have advantages over planartransistor formed using the same materials. In some embodiments, wherethe entire vertical transistor is formed using printing processes thechannel length of the transistor is less than the print resolution(which is unobtainable for a planar transistor whose channel is definedby printing). Generally, the vertical transistors of the presentinvention have shorter channel lengths than are easily obtainable fromthe same processing applied to planar transistors. The verticaltransistors of the present invention with insulating cores allow for thegate to be located on the side of the semiconductor opposite thevertical-support-element, allowing for more design freedom than VTFTsformed with a conductive metal core, and can be fabricated using simpleprinting processes. The following Examples serve to illustrate theadvantages of various embodiments of the present invention and presentprocess.

Printed Top-Gate Vertical Transistors

Top-Gate vertical thin film transistors with polymer cores werefabricated using an all printing process as described above where eachlayer is patterned with the combination of selective area deposition andSALD, where the patterned inhibitor is applied using an inkjet printer.They were formed on 2.5 inch square glass substrates.

Inventive Example I1 One Pixel Width Printed Top-Gate VTFT

To fabricate Inventive Example I1, a glass substrate was provided andcleaned using an O₂ plasma (100 W 0.3 Torr for 1 minute). Next, a 35 wt% solution of SU-8 2010 in cyclopentanone was spun for 10 sec at 500 rpmand ramped to a final spin of 30 sec at 2000 rpm. This coating was curedusing a recipe consisting of a two minute pre-exposure hot plate bake at95° C., a 90 second blanket exposure, a two minute post-exposure hotplate bake at 95° C. and a final hard bake at 225° C. for 5 minutesresulting in a nominal 6000 Å film of cured SU-8.

Next, the surface of the SU-8 was treated with a 30 second O₂ plasma toactivate the surface of the SU-8. The inorganic cap was formed over theSU-8 layer using the combination of SAD and ALD. Selective areadeposition was done using a patterned deposition inhibiting materiallayer using a Fuji Dimatix 2500 piezo-inkjet printer. The inhibitor inkwas a 2 wt % solution of polyvinyl pyrrolidone k-30 (PVP) in diacetonealcohol. The 10 pL cartridge was used with the Dimatix printer, and theprint drop spacing was set to be 70 microns. The PVP ink was printed ina pattern such that the open area defined the pattern of the inorganiccap. Next, 250 Å of Al₂O₃ using DMAI as the metal precursor wasdeposited using the S-ALD system described above and the conditions inTable 1.

After forming the patterned inorganic thin film layer (Al₂O₃) for thecap, the post was formed using a 300 W 0.4 Torr O₂ plasma for 10minutes, resulting in an inorganic cap and a structural polymer postthat together are the vertical-support-element. As shown in the Figures,this results is a vertical-support-element with reentrant profiles 140and 145. The O₂ plasma removed the PVP inhibitor and etched the SU-8 inthe same process step.

Next, the drain and source electrodes were formed by using selectivearea deposition of a conductive inorganic material using an atomic layerdeposition process. This was done by printing a patterned polymericinhibitor such that the inhibitor wicks (that is, moves by capillaryaction) along the reentrant profile in the length dimension of the post.In Example I1, the open area in the inhibitor pattern was designed to be1 pixel wide in the area of the reentrant profile. The same PVP ink andDimatix printer was used in this step as was used in the previouspatterning steps, such that 1 pixel is the equivalent distance of 70microns. The inhibitor wicks, or moves by capillary action, along thereentrant profile from each edge of the open pattern. In order to have afunctional VTFT, the inhibitor ink must meet up, such that the reentrantprofile is completely protected along the width of the transistor. Thepattern for the source/drain electrodes was chosen so that only a singletop-gate VTFT was formed over the vertical-support-element as shown inFIGS. 7a and 7b . In this embodiment the pattern is chosen such that theinhibitor ink is not allowed to fully wick in reentrant profile 145.

After printing the inhibitor pattern containing the open area, 1000 Å ofAZO was deposited using the conditions listed for AZO in Table 1 as thedrain/source electrodes (180, 170). The wicked inhibitor prohibited thegrowth of AZO within the reentrant profile 140, thus forming separateelectrodes via selective area deposition. The inhibitor was removed witha 2 minute 100 W oxygen plasma.

The combination of SAD and ALD was also used to pattern thesemiconductor layer 150. In the present Example I1, nitrogen doped zincoxide (ZnO:N) was deposited as the semiconductor layer 150 at 200° C.,using the conditions listed for ZnO:N in Table 1. Immediately followingthe deposition of the semiconductor layer 150, the same inhibitor wasused to pattern 300 Å of Al₂O₃ for the first layer of a multilayerdielectric stack. DMAI was used as the metal precursor and was depositedusing the S-ALD system described above and the conditions in Table 1.

Next, the dielectric layer was completed by depositing two patternedlayers of 125 Å of Al₂O. For each layer, the inhibitor that was used topattern the previous layer was removed using a 100 W oxygen plasma, thenthe desired new inhibitor pattern was printed using the Dimatix printer,and Al₂O₃ was deposited using DMAI as the metal precursor in the SALDsystem described above and the conditions in Table 1. The combination ofthese patterning/deposition steps provides a gate-dielectric-stackhaving a total thickness of 550 Å.

Both the semiconductor layer 150 and the dielectric layer 130(multi-layer stack) are present at least in the reentrant profile 140.Because Inventive Example I1 was formed on the same substrate asbottom-gate VTFT devices (see discussion of bottom gate VTFT examplesBG1 and BG2 below), the top gate VTFT of Inventive Example I1 hadinhibitor coating the whole device area of the VTFT, which inhibited thegrowth of ZnO:N and Al₂O₃ layers during the deposition of thesemiconductor and buffer layer of the bottom gate devices. The InventiveExample I1, then had the inhibitor removed with a low-power oxygenplasma prior to printing the inhibitor pattern for the gate layer.

The conductive gate layer was provided, using the combination of SAD andALD, providing a gate having 1000 Å of AZO within the reentrant profile.This was done by printing the PVP ink in the gate layer pattern, anddepositing 1000 Å of AZO using the SALD system described above and theconditions in Table 1. The VTFT of Inventive Example I1 was completeafter the deposition of the AZO, and testing was completed withoutremoving the deposition inhibitor material.

Inventive Example I2 Two Pixel Width Printed Top-Gate VTFT

Inventive Example I2 was formed using the same process, and on the samesubstrate, as Inventive Example I1. The only difference betweenInventive Example I2 and I1, is that Inventive Example I2 had a 2 pixelwide pattern used to define the width of the transistor when forming thesource and drain electrodes.

Inventive Example I3 Three Pixel Width Printed Top-Gate VTFT

Inventive Example I3 was formed using the same process, and on the samesubstrate, as Inventive Example I1. The only difference betweenInventive Example I4 and I1, is that Inventive Example I4 had a 3 pixelwide pattern used to define the width of the transistor when forming thesource and drain electrodes.

Electrical testing of Inventive Examples I1-I3 was accomplished by usinga probe station to contact the AZO gate and the two electrodes at thesubstrate level (the first electrode 180 and second electrode 1705 asshown in FIG. 7). The transistors were swept in the linear regime, withthe drain being held constant at 0.2 V (Vd=0.2), and the gate voltagewas swept from −2 V to 6 V. The curves can be found in FIG. 30, and itis clear that the source and drain electrodes are not shorted to eachother, or to the gate of the vertical transistors. Additionally, theexpected increase in current with increasing width of the transistor isobserved.

Inventive Example I4 Two Pixel Width Printed Top-Gate VTFT

Inventive Example I4 is a replicate of Inventive Example I2.

Inventive Example I5 Two Pixel Width Printed Top-Gate VTFT-2 TFTs inSeries

Inventive Example I5 was formed using the same process, and on the samesubstrate, as Inventive Example I2. The only difference betweenInventive Example I5 and I2, is that Inventive Example I2 had a 2 pixelwide pattern that spanned across the vertical-support-element and wasused to define the width of the two vertical transistors connected inseries as shown in FIGS. 1a and 1 b.

Bottom-Gate VTFT BG1: Two Pixel Width Printed Bottom-Gate VTFT

Bottom-gate VTFT BG1 was formed using the same process, and on the samesubstrate, as Inventive Examples I1-I5. The difference between thetop-gate Inventive Examples I1-I5 and the Bottom-gate VTFT BG1 resultfrom the pattern that was used to pattern the thin film layers. Afterforming the vertical-support-element as in Inventive Example I1, theconductive bottom-gate was formed using the combination of SAD and ALD,providing a gate having 1000 Å of AZO within the reentrant profile. Thiswas done in the same SAD-SALD step as the formation of the source/drainelectrodes for Inventive Example I1. Next, when forming thesemiconductor layer and first dielectric layer of Inventive Example I1,the inhibitor was patterned to prevent the growth of ZnO:N and Al₂O₃ inthe area of Bottom-gate VTFT BG1. Next, the dielectric layer forBottom-gate VTFT BG1 was begun by patterning the inhibitor to allow thegrowth of the two patterned layers of 125 Å of Al₂O as in InventiveExample I2. For each layer, the inhibitor that was used to pattern theprevious layer was removed using a 100 W oxygen plasma, then the desirednew inhibitor pattern was printed using the Dimatix printer, and Al₂O₃was deposited using DMAI as the metal precursor in the SALD systemdescribed above and the conditions in Table 1.

After removing the inhibitor from the dielectric pattern, the inhibitorwas applied in the area of the Bottom-gate VTFT BG1 to pattern thesemiconductor layer. Following the patterning of the inhibitor, 150 Å ofAl₂O₃ was deposited as the buffer layer for BG1. DMAI was used as themetal precursor and was deposited using the SALD system described aboveand the conditions in Table 1. Immediately following the deposition ofthe Al₂O₃ buffer layer for BG1, the same inhibitor was used to patternnitrogen-doped zinc oxide (ZnO:N). The semiconductor layer wasdeposited, using the conditions listed for ZnO:N in Table 1. Thecombination of these patterning/deposition steps provides BG1 agate-dielectric-stack having a total thickness of 400 Å.

The device was completed by forming the source/drain electrode in thesame SAD-SALD patterning-deposition step that was used to form theconformal conductive top-gate for Inventive Example I1. The pattern forthe source/drain electrodes was chosen so that only a single top-gateVTFT was formed having a width of 2 pixels. As in the top-gate VTFT, theinhibitor wicks, or moves by capillary action, along the reentrantprofile from each edge of the open pattern. The inhibitor ink meets upsuch that the reentrant profile was completely protected along the widthof the transistor, thus forming two electrically distinct electrodes inthe same deposition step. The Bottom-Gate VTFT BG1 was complete afterthe deposition of the AZO, and testing was performed without removingthe deposition inhibitor material.

Bottom-Gate VTFT BG2: Two Pixel Width Printed Bottom-Gate VTFT-2Vertical TFTs in Series

Bottom-Gate VTFT BG2 was formed using the same process, and on the samesubstrate, as Bottom-Gate VTFT BG1. The only difference betweenBottom-Gate VTFT BG1 and BG2, is that Inventive Example BG2 had a 2pixel wide pattern that spanned across the vertical-support-element andwas used to define the width of two transistors connected in series asshown in FIGS. 8a and 8 b.

Electrical testing of Inventive Examples I4, I5, BG1, and BG2 wasaccomplished by using a probe station to contact the AZO gate and thesource/drainelectrodes at the substrate level. The transistors wereswept in the linear regime, with the drain being held constant at 0.2 V(Vd=0.2), and the gate voltage was swept from −2 V to 6 V. The curvescan be found in FIG. 31, and it is clear that the source and drainelectrodes are not shorted each other, or to the gate of the verticaltransistors. Additionally, it is clear that top-gate and bottom-gateVTFTs can be formed on a single substrate and share multiple layers incommon.

Inverters Formed Over a Common Vertical-Support-Element

In some embodiments of the present inventions two vertical transistorsthat are configured to operate as an inverter can be formed oververtical-support-element. This has advantages over inverters formed fromtwo vertical transistors having independent vertical-support-elements.One clear advantage is a reduction in circuit footprint. Furthermore,the ability to build both top-gate and bottom-gate VTFTs over a commonvertical-support-element allows for the design and fabrication of bothall-enhancement-mode and enhancement-depletion-mode inverters. Thechoice of design is dictated by the circuit requirements for a givenapplication. The specific layout of the inverter, including channeldimensions of the individual transistors and arrangement over thevertical-support-element can vary with circuit requirements includingfootprint. Many designs are possible and are within the scope of thecurrent invention as long as they meet the requirements of having twotransistors formed over a single vertical-support-element, and areproperly connected as having an inverter.

Inventive Example I6 Enhancement-Depletion Inverters Having Two VTFTsOver a Common Vertical-Support-Element

The inverter of Inventive Example I6 was formed using the same process,and on the same substrate, as Inventive Examples I1 and Bottom-gate VTFTBG1. The patterns of the printed inhibitor in the area of the inverterI6 were chosen such that a top-gate VTFT was formed in one reentrantprofile, and a bottom-gate VTFT was formed in the second reentrantprofile of a common vertical-support-element. The layout and structurecan be seen in FIGS. 10a through 10c and FIG. 11. The inverter ofInventive Example I6 has a top-gate VTFT with a thicker gate-dielectric(550 Å) than the bottom-gate VTFT gate-dielectric (400 Å) formed on thecommon vertical-support structure, as shown in FIG. 11. The bottom-gateVTFT has a channel width of 2 pixels (nominally 140 microns) and achannel length that is determined by the profile of the wickedinhibitor. The channel length is shorter than the obtainable printedfeature size, and varies as a function of the width of the VTFT (thedistance the inhibitor is required to wick). The 2 pixel bottom-gateVTFT of the inverter of Inventive Example I6 has an approximate averagelength of 25 microns (while the minimum printed feature is greater than70 microns). The top-gate VTFT has a channel width of 1 pixel, and anapproximate average length of 50 microns.

Inventive Example I7 Enhancement-Depletion Inverters Having a VTFT and aLong-Channel Top-Gate VTFT Connected Over a Vertical-Support-Element

The inverter of Inventive Example I7 was formed using the same process,and on the same substrate, as inverter of Inventive Example I7. Thepattern of the printed inhibitor in the area of the inverter I7 waschosen to form a bottom-gate VTFT in the first reentrant profile of avertical-support-element, and a top-gate planar TFT having a channelwith a portion extending in a direction orthogonal to the substrate inthe second reentrant profile. The structure I7 should be understood fromthe cross-sectional view of FIG. 27, with the additional elementsrequired to form a top-gate planar TFT having a thicker gate-dielectric(550 Å) than the bottom-gate VTFT gate-dielectric (400 Å). As shown, thechannel length of the long-channel top-gate VTFT is primarily defined bythe portion of the channel that is parallel to the substrate. Theequivalent circuit for inverter I7 is the same as that of I6. Thebottom-gate VTFT on inverter I7 is equivalent to the bottom-gate VTFT ofI6, and has a channel width of 2 pixels (nominally 140 microns) and anapproximate average channel length of 25 microns. The long-channeltop-gate VTFT has a channel width of 2 pixels, and a nominal channellength of 80 microns. The channel length of the long-channel top-gateVTFT was defined by printing a 2 pixel by 1 pixel feature adjacent tothe second reentrant profile. In this way, the channel dimensions aredefined by printing and are much larger than could be obtained bywicking the inhibitor into the reentrant profile (80 microns>25microns). Furthermore the portion of the channel of the long-channeltop-gate first transistor (first semiconductor channel) that extends ina direction parallel to the substrate is approximately 130 times greaterthan the height of the vertical-support-element (channel length=80microns, vertical-support-element height=0.6 microns).

Electrical testing of Inventive Examples I6 and I7 was done using aprobe station to contact the AZO Vdd, ground, Vin and Vout nodes of theinverters. The response of Vout as a function of Vin, for Vdd=4 V can beseen in FIG. 32. As shown, both Inventive Examples form functionalinverters. The performance of the inverters of the present invention canbe tuned by changing the channel dimensions of either the drive or loadTFT, or by varying the gate-dielectric thickness for each TFTindependently. Although FIG. 32 illustrates the response of anenhancement-depletion mode inverter from two transistors—namely atop-gate one load TFT and a bottom-gate drive TFT—from the previousdescription it should be clear that dual-gate inverters having similarfeatures could be formed.

Mixed Circuit Experiments

The electronic devices of the present invention have advantages overdevices formed with a single device architecture. By using processesthat allow for the easy fabrication of vertical and planar transistorson a common substrate, the size of the transistor can be varied byorders of magnitude without a corresponding impact on circuit footprint.Furthermore, the use of thin film metal-oxide allows the varioustransistor architectures to be fabricated from a common set of materiallayers. The use of selective area deposition brings further advantagesin patterning, including the independent control of the gate dielectricthickness for elements on a common substrate.

Comparative Example C1 Enhancement-Mode Ring Oscillator

Comparative Example C1 is a 7-stage enhancement-mode ring oscillatorformed from all planar bottom-gate TFTs. Comparative Example C1 wasfabricated in a similar manner as Inventive Example I1, using thecombination of spatial ALD and selective area deposition (SAD).Comparative Example C1, did not have a vertical-support-element formedon the substrate. Each transistor in the ring oscillator had a gatelayer of 1000 Å of AZO, gate-dielectric of 750 Å of Al₂O₃, 200 Å ofZnO:N for the semiconductor layer and 1000 Å AZO for the source anddrain electrodes. As with Inventive Example I1, the printed inhibitorused to pattern each of the device layers was a 2 wt % polyvinylpyrrolidone (PVP) k30 in diacetone alcohol solution which was printedusing a Dimatix 2500 printer. The printing was done using the 10 pLcartridge which resulted in a spot size on the substrate ofapproximately 90 microns under typical operating conditions. Thepatterns were printed at a pixel-to-pixel distance of 70 microns (363dpi). The pattern of PVP used to pattern the AZO layer in the source anddrain pattern was left on the device during testing, such that the backchannel of the semiconductor layer was covered by a thin layer of PVPk30.

Each inverter of the ring oscillator was connected as the equivalentcircuit shown in FIG. 26a . The drive transistors for each inverter hadan as designed W=12 pixel/L=1 pixel, and the load transistors for eachinverter had an as designed channel of W=2 pixel/L=5 pixel. The samplewas analyzed by using a probe station to contact the AZO Vdd, ground,and output nodes of the ring oscillator, and the oscillator wascharacterized as a function of Vdd (Input Voltage). The time per stageas a function of Input Voltage can be found in FIG. 33.

Inventive Example I8 Enhancement-Mode Ring Oscillator with VTFT DriveTransistors and Planar TFT Load Transistors

Inventive Example I8 is a 9-stage enhancement-mode ring oscillatorformed from the combination of vertical and planar bottom-gate TFTs.Inventive Example I8 was fabricated in a similar manner as Bottom-GateVTFT BG2, using the combination of spatial ALD and selective areadeposition (SAD). The vertical-support-element was formed as inBottom-Gate VTFT BG2, and the SU-8 structural polymer was removed fromthe substrate in the area of the planar bottom-gate load transistors.Each transistor in the ring oscillator had a gate layer of 1000 Å ofAZO, gate-dielectric of 400 Å of Al₂O₃ (250 Å primary dielectric layerand a 150 Å buffer layer), 200 Å of ZnO:N for the semiconductor layerand 1000 Å AZO for the source and drain electrodes.

The inverters of the ring oscillator were configured as illustrated inFIGS. 25a through 26c , where the drive transistor of a typicalenhancement-mode inverter is replaced with two bottom-gate VTFTs thatare connected in series. The two vertical drive transistors each had achannel width of 2 pixels, and a channel length that was defined by thewicked inhibitor along the reentrant profile (approximate average lengthof 25 microns). The planar bottom-gate load transistor had an asdesigned channel of W=2 pixel/L=5 pixel as in Comparative Example C1.Inventive Example I8 was characterized like Comparative Example C1; thetime per stage as a function of Input Voltage can be found in FIG. 33.

Inventive Example I9 Enhancement-Mode Ring Oscillator with VTFT DriveTransistors and Planar TFT Load Transistors

Inventive Example I9 is a 9-stage enhancement-mode ring oscillatorformed from the combination of vertical and planar bottom-gate TFTs.Inventive Example I9 was fabricated in the same manner and on the samesubstrate as Inventive Example I8. Configuration and layer thickness ofthe ring oscillator of Inventive Example I9 are equivalent to those ofInventive Example I8, with the exception of the dimensions of the driveand load transistors. The two vertical drive transistors each had achannel width of 4 pixels, and a channel length that was defined by thewicked inhibitor along the reentrant profile (approximate average lengthof 4 microns). The planar bottom-gate load transistor had an as designedchannel of W=2 pixel/L=2 pixel. Inventive Example I9 was characterizedlike Comparative Example C1; the time per stage as a function of InputVoltage can be found in FIG. 33.

The time per stage as a function of Input Voltage for ComparativeExample C1 and Inventive Examples I8 and I9 can be seen in FIG. 33. Asshown, both Inventive Examples I8 and I9 form functional ringoscillators with shorter time per stage than the all-planar TFToscillator of Comparative Example C1. As shown, the performance of thering oscillators of the present invention can be tuned by changing thechannel dimensions of either the drive or load TFT, or by varying thegate-dielectric thickness for each TFT independently. Inventive ExampleI9 has a stronger drive and load transistor configuration and gives ashorter time per stage. Both Inventive Example I8 and I9 have a smallerfootprint than Comparative Example C1, even though they have a largernumber of stages.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   1 planar bottom-gate TFT-   2 planar bottom-gate TFT-   3 planar bottom-gate TFT-   4 planar bottom-gate TFT-   5 planar bottom-gate TFT-   6 planar bottom-gate TFT-   12 gate-   13 dielectric layer-   14 via region-   15 semiconductor layer-   18,19 source, drain-   20 post-   21,22 dielectric layer-   23 multilayer dielectric stack-   25 additional protection dielectric layer-   26 buffer layer-   30 cap-   91 exhaust channels-   92 metal precursor flow-   93 oxidizer-containing flow-   94 output face-   95 nitrogen purge flow-   96 delivery head-   97 example substrate-   98 arrow-   99 gap-   100 top-gate vertical transistor-   102 top-gate vertical transistor-   103 top-gate vertical transistor-   104 top-gate vertical transistor-   105 top-gate vertical transistor-   106 top-gate vertical transistor-   107 top-gate vertical transistor-   108 top-gate vertical transistor-   110 substrate-   115 second conformal dielectric layer-   120 vertical-support-element-   125 conformal conductive gate layer-   125 a, 125 b conformal gate-   127 gate-   130 dielectric-   131 multilayer dielectric stack-   132 dielectric-   135 via-   140 first reentrant profile-   141 first reentrant profile-   145 second reentrant profile-   146 second reentrant profile-   150 semiconductor-   155 semiconductor-   170 second electrode-   175 third electrode-   180 first electrode-   181 first electrode-   185 fourth electrode-   200 top-gate vertical transistor-   202 top-gate vertical transistor-   203 top-gate vertical transistor-   204 top-gate vertical transistor-   205 top-gate vertical transistor-   206 top-gate vertical transistor-   207 top-gate vertical transistor-   303 top-gate vertical transistor-   304 top-gate vertical transistor-   305 bottom-gate vertical transistor-   306 bottom-gate vertical transistor-   315 bottom-gate vertical transistor-   316 bottom-gate vertical transistor-   325 bottom-gate-   327 bottom-gate-   330 dielectric-   335 dielectric-   350 semiconductor-   370 second electrode-   375 third electrode-   380 first electrode-   385 fourth electrode-   410 planar bottom-gate TFT-   411 planar top-gate TFT-   413 planar bottom-gate TFT-   414 planar top-gate TFT-   415 planar bottom-gate TFT-   416 long channel bottom-gate VTFT-   417 long channel top-gate TFT-   418 top-gate VTFT-   420 bottom-gate VTFT-   421 bottom-gate VTFT-   423 top-gate VTFT-   424 top-gate VTFT-   425 bottom-gate VTFT-   426 bottom-gate VTFT-   427 bottom-gate VTFT-   428 bottom-gate VTFT-   430 bottom-gate VTFT-   431 bottom-gate VTFT-   433 top-gate VTFT-   434 top-gate VTFT-   435 bottom-gate VTFT-   500 a,b,c width of top-   505 a,b,c minimum width defining reentrant profile-   510 a,b,c first side-   520 a,b,c second side-   530 a,b,c height of vertical-support-element-   535 a,b,c top of vertical-support-element-   540 a,b,c first reentrant profile-   545 a,b,c second reentrant profile-   701 dual-gate vertical transistor-   702 dual-gate vertical transistor-   703 dual-gate vertical transistor-   704 dual-gate vertical transistor-   705 dual-gate vertical transistor-   706 dual-gate vertical transistor-   715 dielectric-   720 conformal bottom gate-   721 conformal bottom gate-   725 conformal top gate-   727 conformal top gate-   730 dielectric-   735 via-   750 semiconductor-   751 semiconductor-   760 electrically conductive gate structure-   770 second electrode-   775 third electrode-   780 first electrode-   785 fourth electrode-   801 bottom-gate vertical transistor-   802 bottom-gate vertical transistor-   803 bottom-gate vertical transistor-   804 bottom-gate vertical transistor-   805 bottom-gate vertical transistor-   820 electrically conductive gate structure-   825 gate layer-   830 dielectric-   850 semiconductor-   870 second electrode-   875 third electrode-   880 first electrode-   885 fourth electrode-   900 planar top-gate TFT-   910 substrate-   920 top-gate-   930 dielectric-   950 semiconductor-   980.985 source/drain-   A,A′-   C1,C1′-   C2,C2′-   C3,C3′P,P′-   d

The invention claimed is:
 1. An electronic device comprising: asubstrate; a vertical-support-element on the substrate, thevertical-support-element extending away from the substrate, thevertical-support-element including a first edge having a first reentrantprofile; a conformal semiconductor layer in contact with thevertical-support-element in the reentrant profile; a first electrode incontact with a first portion of the semiconductor layer, the firstelectrode located over a top of the vertical-support-element; a secondelectrode in contact with a second portion of the semiconductor layer,the second electrode located over the substrate and not over thevertical-support-element, and adjacent to the first edge of thevertical-support-element; a conformal dielectric layer on the conformalsemiconductor layer in the reentrant profile; and a conformal conductivegate on the conformal dielectric layer in the first reentrant profile,wherein the first electrode and the second electrode define asemiconductor channel of a transistor.
 2. The electronic device of claim1, wherein the vertical-support-element comprises: a polymer post havinga height dimension extending away from the substrate to a top, the firstedge of the vertical-support-element located along the height dimensionof the post; and an inorganic material cap located on the top of thepost and extending beyond the first edge to define the first reentrantprofile.
 3. The electronic device of claim 2, wherein the inorganicmaterial cap includes at least one of a Al₂O₃, Sift, HfO, ZrO, TiO₂,Ta₂O₅, Si_(x)N_(y), ZnO, and a doped-ZnO material.
 4. The electronicdevice of claim 2, wherein the cap extends beyond the edge of the postby a distance that is less than the height of the post.
 5. Thevertical-support-element of claim 1 further comprising: a secondconformal dielectric layer covering the VTFT structure and a portion ofthe substrate, the second conformal dielectric layer located between thesemiconductor layer and the VTFT structure.
 6. The electronic device ofclaim 1, the first electrode and the second electrode are separated by adistance, wherein the distance between the first electrode and secondelectrode is greater than zero when measured orthogonally to thesubstrate surface.
 7. The electronic device of claim 1, thevertical-support-element including a second edge and a second reentrantprofile, the device further comprising: a third electrode located incontact with a third portion of the semiconductor layer over thesubstrate and not over the vertical-support-element, and adjacent to thesecond edge, wherein the first electrode and the third electrode definea second semiconductor channel of a second transistor.
 8. The electronicdevice of claim 1, the vertical-support-element including a second edgeand a second reentrant profile, the device further comprising: a thirdelectrode located in contact with a third portion of the semiconductorlayer over the substrate and not over the vertical-support-element, andadjacent to the second edge; a fourth electrode located in contact witha fourth portion of the semiconductor layer over the top of thevertical-support-element, and not in contact with the first electrode;and a second conductive gate on the conformal dielectric layer over thesecond edge of the vertical-support-element in the second reentrantprofile, wherein the third electrode and the fourth electrode define asecond semiconductor channel of a second transistor, the transistor andsecond transistor being independently operable.
 9. The electronic deviceof claim 1, wherein the first electrode and the second electrode includetransparent conductive oxide material.
 10. The thin film transistor ofclaim 1, wherein the conformal conductive gate layer, the conformaldielectric layer, the conformal semiconductor layer, the firstelectrode, and the second electrode each include a metal oxide material.11. The electronic device of claim 1, wherein thevertical-support-element includes a polyester, polyetherester,polyamide, polyesteramide, polyurethane, polyimide, polyetherimide,polyurea, polyamideimide, polyphenyleneoxide, phenoxy resin, epoxyresin, polyolefin, polyacrylate, polyethylene-co-vinyl alcohol, acopolymer thereof, or a mixture thereof.
 12. The electronic device ofclaim 1, wherein the vertical-support-element has a height dimensionextending away from the substrate that is less than or equal to 10microns.
 13. The electronic device of claim 1, wherein the firstelectrode and the second electrode have the same material compositionand layer thickness.
 14. The electronic device of claim 1, the firstelectrode having a first end and the second electrode having a secondend, wherein the first end of the first electrode and the second end ofthe second electrode are vertically aligned.
 15. The electronic deviceof claim 1, wherein the semiconductor channel defined by the firstelectrode and the second electrode has a length dimension that is lessthan 10 times the height of the vertical-support-element.
 16. Theelectronic device of claim 1, the semiconductor channel defined by thefirst electrode and the second electrode including a width dimension anda length dimension, wherein the length dimension varies along the widthdimension of the transistor.
 17. The electronic device of claim 1,wherein the conformal conductive gate layer is only located in thereentrant profile adjacent to the semiconductor channel.